To answer your questions (short version):
Did the Haitian Revolution affect slaves in Brazil:
Yes! Brazilian slaves were granted the condition to own their own slaves (twisted, no?) during the revolutionary period by the master class. The master class feared their slaves would rebel, forcing the 'elite' to recognize their chattels' (slaves) personhood by granting property-owning concessions. Rebellions were extremely bloody and masters wanted to gain their slaves' allegiance and loyalty.
It is no coincidence that Brazilian slave ownership of slaves escalated during and following the period of the Haitian Revolution. The documentation is rampant in the testaments (wills) of the enslaved and formerly enslaved.
The world is much more connected than it seems.
To what extent were Haiti and Brazil connected during the revolutionary period? That is to be determined (more on that below).
Why isn't there much research on the topic?
It is expensive to conduct such a study. And there are few people equipped to take on such a monumental task.
Extended answer:
What is known from the few resources available is that anxiety among the slaveholding elite in Brazil became especially pronounced during and after the revolution that overturned the slave regime in Saint-Domingue (present-day Haiti) (see, Julius Scott III, The Common Wind). Julius Scott’s study followed maritime slaves traveling throughout the Caribbean and “news currents" (enslaved maritime workers communicated with one another along trade routes, spreading information about rebellions). Indeed, there were slaves operating in the Caribbean during the Haitian Revolution that either connected with Brazilian maritime workers or traveled directly to the shipping ports in Rio de Janeiro and Salvador. Unfortunately, Scott’s dissertation only relied on English sources to connect Afro-American news currents. Thus, a study has never been fully applied to the Brazilian context.
To what extend did the slave rebellions in distant lands affect the slaves in Brazil?
To my knowledge, Brazilianist Luiz Mott is the only historian commenting directly on the connection between slave rebellions in Haiti and Brazil. In an 1805 document, Mott provides evidence that Afro-Brazilians in Rio de Janeiro wore necklaces displaying the image of Dessalines (Afro- leader in the Haitian Revolution) within a year of Saint Domingue’s independence (see, Luiz Roberto de Barros Mott, “A escravatura: a propósito de uma representação a El Rei sobre a escravatura no Brasil").
Although the subject needs considerable attention from scholars, slaves and freedpeople working in maritime networks (shipping) connected Afro- populations throughout the greater Atlantic World. Salvador, Brazil's position as one of the most heavily trafficked ports during the nineteenth century contributed to the anxiety of the slaveholding class; the elite feared maritime laborers would spread revolutionary discourse (propaganda) encouraging large slave insurrections (see, Washington Santos Nascimento, “Além do medo: a construção de imagens sobre a revolução hatiana no Brasil (1791-1840)”).
Tension among the slaveholding class was especially heightened during the 1807-1835 period in Bahia (Salvador, Brazil), which opened with a stillborn plot among Mandinka and Huasa blacks and concluded with a primarily Yoruba (Nagô) insurrection (see, Eugene D. Genovese, Roll, Jordan, Roll). In the interim, five slave uprisings threatened to overturn the slave regime in Salvador, Brazil.
The prospect of an overturned slave regime horrified the master class of Salvador, Brazil. Masters responded to the revolutionary climate by granting concessions – dangling “freedom” in front of their slaves – because to do so meant to foster loyalty among the slaves, and simultaneously protect their lives and interests (see, João José Reis, Slave Rebellion).
It was during times of distant rebellion (revolutions in the Caribbean) that the master class in Brazil also sought to manipulate the slave system to earn the slaves' loyalty. Thus, during the Haitian Revolution, slave ownership among Brazilian slaves and the 'emancipated' surged (see, Mieko Nishida, Slavery and Identity). Coincidence? To do so meant that masters recognized their chattels' personhood while simultaneously benefitting from their slaves’ loyalty due to concessions granted.
Brazil is one of the only known places with extensive records linking slaves and freedpeople (formerly enslaved) to slave ownership. Slave ownership among Afro- populations increased dramatically during the revolutionary period.
Imagine going to Brazil in the nineteenth century and seeing slaves owning slaves. Yep.
To explore the second part of your question: Why have so few historians attempted to connect the events between slave-owning places?
Your question is about the direction of slave studies in the greater Atlantic World. Recently, scholarship has taken a new turn in the emerging field of ‘international history’. Historians speaking multiple languages are beginning to approach the archive (where historians conduct research and look at primary sources) from a new lens, distancing scholarship from previous regional studies (studying slavery in one location).
Until the 1990s, studies related to Brazilian slavery were regional in scope. Brazilianists are now attempting to 'connect' the archive by approaching studies in international frameworks (connecting sources from multiple locations, spanning across borders). To do so would mean scholars need to decipher primary sources in multiple languages and visit archives in distant, connected places.
Historians operating in the field of 'international history' are required to speak, train, and read sources in multiple languages. Your question is directly related to the skills and resources historians are required to have as scholarship expands in scope, moving from a regional to an international lens.
To directly answer your question: Can you say - "cha ching"? It is expensive to visit archives and conduct research in multiple countries. Moreover, very few people already have the required skillset and time to complete the research.
Julius Scott III's study of maritime slaves in the Caribbean during the Haitian Revolution encourages historians to approach the topic from an international perspective. To be a maritime slave meant that news traveled among currents from one slave-owning place to another. No longer can slave histories in the Atlantic World be viewed in isolated units; a revolution in Haiti would have consequences in distant places like Brazil.
There is much to be uncovered about your question. Perhaps this inquiry is the subject of a future PhD dissertation.
Sources:
I am a journalist and Brazilianist currently applying for a PhD in History. I have my MA and MSc in International History. Feel free to shoot me a message.
Brazil is already covered, but as you seem interested in broader examples and only used that as one possibility, this older answer about the USA might also be of interest.