Not revealing a Moat to an attack when the attack will hurt you (or Bane to Young Witch)
Discard attacks in anticipation of Menagerie or draw to X (e.g. Jack). I've allowed Pirate Ship to attack to intentionally trash Coppers. Margrave when one card will save a completely dead hand. Soothsayer when I want the card really badly. Haunted Woods when I want to take advantage of the top-decking.
Any attack you want to take, really. There are so many reasonable things here that when they apply you are probably not even going to be buying Moat in the first place. Which makes them edge-cases with optimal play, I guess, because you have to actually have a Moat in your hand, not just in the kingdom.
Not revealing a Province when an opponent plays Tournament
I have done this exactly one time years ago to trigger a bad reshuffle for my opponent.
Not revealing Horse Traders to a discard attack when discarding won't help you
Haven't done this, but not triggering a bad reshuffle could be a valid reason.
Anyone who can add to the list of "ridiculous" plays, or has an edge case for them?
Not triggering a reshuffle is usually a good reason to do something strange. Presence of Events/Landmarks is another ("you bought a Curse?!"). Outside of that....
Playing Possession on a Possession turn because your hand is junk and you want it discarded (a very expensive, round-a-bout Guide).
Playing but not discarding a Treasure for Stables, not trashing for Spice Merchant, etc. when you have a valid treasure because you need another Action in play and you need the Treasure to buy something (e.g. Emporium, Horn of Plenty).