A more general approach to shared memory via lockable heaps is a feature Nim seems will implement soon.
I'm interested in reading more about it, because Nim is doing a lot of experimental stuff and it's always interesting to look at its designs.
(Edited to remove speculation about how well it will perform before reading about it.)
Note that shared, lockable heaps need not be heavyweight structures. It is entirely possible to imagine a shared hash table with one heap per bucket and fine-grained locking, for example. Collections for such small heaps can be fast because the number of roots is limited, and (depending on what invariants you guarantee), you can even forgo stack scanning for most collections or limit the number of stack frames that need to be traversed.
At the programming language level, this then mostly involves maintaining mutual exclusion (in Eiffel, the necessary semantics are attached to how "separate" types are handled) and having the optimizer get rid of unnecessary copies.