I don't think PostgreSQL is going to work for you if thats a requirement. A tablespace doesn't contain the metadata for the items in it, thats stored in the main cluster space (pg_catalog, etc)
Would it be difficult to export the metadata as part of the shutdown process of the service? Or am I simply reaching a bit too much? (For the record, my application exists because I run into this type of "things don't work that way" and than I find some interesting solutions <grin> But maybe this time I am simply better off with Firebird.)