Joseph Sieyès "What is the third estate"

Depending on your definition of "prior to the French Revolution" this might be exactly what you are looking for. ( However, he was a clergyman (2nd estate) rather than a member of the first). His writing helped prompt the start of the revolution, so to that extent he was warning before, yet at same time he was writing on the eve of the events.

After skimming the article you posted what Sieyès had said seemed similar. I would recommend reading through it and seeing if it helps with your question.

His piece can be accessed for free from the Internet Modern History sourcebook at http://www.fordham.edu/HALSALL/mod/sieyes.asp

The early stages of the French Revolution were pretty much entirely elite-led, comprising a significant amount of aristocrats. Many of the members of the National Assembly / Third Estate who set the stage for the French Revolution were aristocrats themselves; This guy (http://en.wikipedia.org/wiki/Honor%C3%A9_Gabriel_Riqueti,_comte_de_Mirabeau), a noble, was their head at the time of the Tennis Court Oath. The third estate had many supporters, and many of the revolutionary leaders were members of the nobility and the clergy. It was not until the Reign of Terror that we can say that the French Revolution really cut its ties entirely with the French aristocracy. The only source I have with me is Georges Lefebvre's The French Revolution and he categorically states that "The French Revolution was started and led to victory in its first phase by the aristocracy". He then traces the movement away from the aristocratic revolutionary leadership towards the bourgeoisie, and then Jacobin populism (what we tend to think of as the revolution proper).

On the specifics of your question, I think that it is important to recognise that the political debate about Revolution in France shifted radically. The early (aristocratic) revolutionaries furiously discussed ideas of political organisation- such as how much power each section of government should have etc. The key notion which guided their debate was liberty, similar to how it was understood in classical times, and their aim was principally to create a non-tyrannical political system (similar, in a sense, to America). It was not until the French Revolution became a popular revolt that the question of 'social equality' really became a core principle and aim of government. Thus, I think you would be hard pressed to find aristocrats who engaged with the notion of social equality, as the change in debate was so sudden and sharp. Hannah Arendt in On Revolution refers to this transition as the emergence of the 'Social Question'.