I was on the reverse routing same night (ORD-DCA late flight) that also cancelled wx. Was able to get EXPLAT desk to interline me on UA to Dulles but it did take the agent some time to call and get approval. It can be done, takes an agent who wants to help and ensuring your ticket change is marked as INVOL. But as with everything with the 'New American' aka USAir dba AA, YMMV.