adding the three lines in the first code block here
https://wiki.gentoo.org/wiki/Hardened/GNU_stack_quickstart#Patching#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
to common/arm/asm.S from bullseye's x264 source package got me a libx264-160 deb that allows ffmpeg to launch without a permission denied (executable stack) error within Bullseye in UserLAnd on non-root Android (armv8l).
I have not yet checked whether x264 codec functionality works properly, nor if this fix applies to newer versions or upstream.