If you like this tool, you might also be interested in reptyr, which lets you reparent a process to a different tty.
https://github.com/jerome-pouiller/reredirect/
Ended up using dtach. Needs to be run ahead of time, but very direct and minimal stdin/stdout piping tool that's worked great with everything I've thrown at it. https://github.com/crigler/dtach
also vmux appears to be specifically tailored to vim/neovim