I don't know if this is even possible, but a good travel agent can find out (if he/she doesn't already know) and tell you. I'd probably just book these as separate trips, buying a train ticket that had some protection if you miss the train because your flight arrives late.

I can't imagine a fare rule that would limit your ability to take trains while at a destination you reached by air. If there were such rules, the Heathrow Express and the western end of the Piccadilly Line wouldn't be nearly as busy as they are.