[...] assuming the rest of idea works out
well, we'd eventually like to move to a tiered model where the GIL
becomes a read/write lock. Most code execution in subinterpreters
would then only need a read lock on the GIL, and hence could happily
execute code in parallel with other subinterpreters running on other
cores.

However, that aspect of the idea is currently just hypothetical
handwaving that would need to deal with (and would be informed by) the
current work happening with respect to the GILectomy, as it's not
particularly interesting as far as concurrency modeling is concerned.

By contrast, being able to reliably model Communicating Sequential
Processes in Python without incurring any communications overhead
though (ala goroutines)? Or doing the same with the Actor model (ala
Erlang/BEAM processes)?

Those are *very* interesting language design concepts, and something
where offering a compelling alternative to the current practices of
emulating them with threads or coroutines pretty much requires the
property of zero-copy ownership transfer.