Just thinking out loud: with 1 Bishop and all Rats for the rest of your hand, you could guarantee 3VP every turn (trash a Rat, buy a copper or curse, keep playing Rats until you hit the Bishop, eat up any coppers/curses along the way, rinse and repeat), until the Rats run out (but that's too slow).
With 2 Bishops and 1 or more Worker's Villages and Rats for the rest of your hand, then your play could be:
1) Your hand is Bishop only + Rats: Trash a Rat for 3VP, buy a copper or a curse.
2) Your hand is Worker's Village + Rats: play it and get a +buy, keep playing Rats until you hit both (or more) Bishops: 6VP, buy 2 or more coppers to feed your Rats.
3) Your hand is all Rats: keep playing Rats until you get to 1) or 2).
You could collect at least 3VP every turn until all the Rats run out. If you managed to trash all 20 Rats that's 60VP. With 2 Bishops in play you could conceivably buy out all the estates along with the Rats and Curses for 3-pile.
Pawn and Hamlet would also work well for this to get +Buy. You'd never need to buy a $5 card.