wl-memory now has a much richer data model which allows for tiered metadata storage (at the adapter, datastore, and collection level).  This doesn't necessarily have to be global - could be that wlmemory gets thrown in a wrapper that gives its otherwise context-free methods closure scope on an optional one-off local variable.  But even then, that probably isn't a good idea since its non-intuitive and not the way that any other db works.