Alternatively:

4. Do the waitpid in a separate thread (needs work-around for the
multi-threaded fork problem, probably C things? Or modifying Guile and
maybe glibc to avoid async-unsafe things or make more things
async-safe or whatever the appropriate ...-safe is here.)

For shepherd, multithreading is not an option due to the semantics of
fork in the presence of threads.

From what I've read, multi-threaded fork is safe as long as you do an exec 'immediately' afterwards, without doing things like taking locks or allocating memory with malloc in-between the fork and exec. I don't think it's possible to do that in Guile code, but that's what the C things are for.