I wish to create an alter command which will allow a table to have OIDs added or removed.
The tricky part appears to be changing the tuples themselves. I believe if I pull the same trick that cluster does (create new file, copy tuples, etc) it can be done fairly easily.
First, set up pg_class appropriately (oid flag).
Second, copy out tuples from oldfile to newfile, running a heap_deformtuple() -> heap_formtuple() process on each. Since heap_deformtuple only deals with positive numbered attributes (non-system attributes) this should be safe to do on a mis-configured relation. heap_formtuple completes the dirty work of setting up the OID column appropriately.