1) What happens if I play BoM or Overlord as encampment?
2) What happens if I play Procession on encampment?
1) Exactly the same thing as if you play a regular Encampment... you get +2 cards, +2 actions, then you may reveal a Gold or Plunder from your hand. If you do, nothing else happens, BoM-as-Encampment will be cleaned up like normal and become a BoM again when discarded. If you do not, then you will set it aside (where it immediately becomes a BoM again), and still return it (the BoM) to the supply (onto the BoM pile).
2) If you didn't reveal a Gold or Plunder either of the 2 times you played it, then it will get set aside and thus fail to trash due to lose-track. You'll still get an action card costing
more, because that's not dependent upon trashing.