Is there something particular about the history of these two countries that facilitated this cultural transmission, or does the Belgian terror feature in the cultural memory accross the continent?
I know Liberia and Sierra Leone were both used for commercial rubber production, was there something about rubber production that made it uniquely terroristic?
First off, I think it is important to note that the brutality unleashed in the Congo Free State period from 1870-1908 shouldn't really be conflated as "Belgian colonial terror". The International Association for the Congo and Leopold II as the chairman existed as legally separate from his role as king of the Belgians. That is to say, from 1870-1908 the Congo Free State existed as a private fief of Leopold, not as a Belgian colony. To be sure, a majority of the officers involved in the Force Publique and administrators came from Belgium, but there were also individuals from the US (Henry Morton Stanley) the UK (Joseph Conrad, Joseph Moloney), Canada (William Grant Stairs) Italy, Sweden, Denmark, and other countries who served in the Force Publique or as administrators.
That is not to say that Belgian colonial rule from 1908-1960 was completely benign, or that the Belgians did not interfere in the Congo Crisis in the 1960s. Just that the Congo Free State was legally and practically different from the Belgian Congo.
So, I think you are assuming a connection when there doesn't have to be one. Contemporary reporting of the Sierra Leone civil war speculated that a 1999 campaign slogan "the future is in your hands" may have inspired RUF rebels to begin amputating hands to discourage people from voting, and the practice continued to deny civilians the ability to harvest crops or mine diamonds that might support the central government.
I had not heard that amputations occurred in the Liberian civil war, I only heard of it in reference to Sierra Leone.