Basically I think there should be a "coprocess protocol" that makes persistent processes look like batch processes, roughly analogous to CGI or FastCGI.

I thought that could be built on top of bash, but perhaps that's not possible.

I'll need to play with it a bit more. I think in bash you can have named multiple coprocesses with their descriptors stored in an array like ${COPROC[@]} or ${MY_COPROC[@]}. But there are definitely issues around process lifetimes, including the ones you point out. Thanks for the feedback.