I had a similar question to the ones above but with one more twist.
I am going IAD - LHR (UA) then LHR - BKK (TG) on one FF ticket.
I then need to purchase a separate ticket to go BKK - REP, possibly on Bangkok Air. You all have answered my question about bags interlining TG to PG, but do you think United will be able to book them all the way through since PG is not *A and they probably have no agreement with them?