It's been quite a long time since I played with this patch so my memory might be a bit fuzzy.  As I recall, it sounds good in theory but in practice it doesn't really work.  One of the core problems is that many extension modules keep references to Python objects in global or static variables.  These references keep pretty much everything alive and prevent GC cleanup of modules.

So, a necessary condition to this working is to get rid of those references and use the new module struct facility introduced by Martin.  That would be a huge amount of work but I think should be the long term goal.