I think it can't use the same recipe. Sam's approach for CPython uses biased reference counting. Internally, Pypy uses a tracing garbage collector, not reference counting. I don't know how difficult it would be to make their GC thread-safe. Probably you don't want to "stop the world" on every GC pass so I guess changes are non-trivial.

Sam's changes to CPython's container objects (dicts, lists), to make them thread safe might also be hard to port directly to Pypy. Pypy implements those objects differently.