There was Condor for this[1], a couple of decades ago. Condor would checkpoint the process and restart it on another machine entirely user-level (but requiring processes to link to their library) by continuing to forward system calls. It of course had plenty of limitations, and some of their decisions would be considered serious security risks now (e.g. they intercept open() and record the name, and assume that its safe to reopen a file by the same name after migration), but it was an interesting system.

I think migrating cooperating processes would be fairly simple, and the big challenge is rather to decide on the right set of tradeoffs.

[1] https://chtc.cs.wisc.edu/doc/ckpt97.pdf