Off topic, but I'm just asking someone who reads Russian to tell me whether Bakhtin's "material bodily lower stratum" in the Rabelais book couldn't have been translated "physiological substrate".

Well, sure it could have; it could be translated any number of ways. Your version isn't exact, but it gets the idea across, and the exact rendition is hopelessly clunky, as discussed in the comments to this post.

I either missed or forgot the original answer.

That unfortunate translation virtually ruined the Rabelais book for me.

"How many angels can dance on the head of a pin?" --> "Does rationality require a material substrate?"