> What modifications are needed to run through Fil-C, if any?
Most likely none.
The most likely bugs you'll encounter are due to Fil-C using musl, not glibc (that always leads to some incompatibilities for GNU code).
> Are you subsetting the language
No.
> or do those libraries/applications run into some "benign" UB under normal operation that is caught by Fil-C?
Sometimes, but rarely.