At one time I thought we'd try doing what the JavaScript V8 engine behind Chrome/Node/etc. uses, something called "isolates":
But the direction I imagine going in now is more like Go, e.g. "green threads" as a more cooperative way of letting the interpreter shuffle state around:
What Color is Your Function? – journal.stuffwithstuff.com
Switching to Stackless: Why this, Why now?
So concurrency would be managed within the interpreter "service" itself--not as something that you'd have to do "locks" in native C code to surround on different threads. This would allow for concurrency...though (probably) only one CPU core would be running the interpreter code; just shuffling between whichever tasks/threads that were requesting to use it.
I'm in constant transit, currently in a search for a new place to live, and that's taking up a lot of time. So I don't really know what the timetable is on stackless, or much of anything at this point I'll see about getting around to my New Years status post shortly.