With cards that can save trashed cards, I've thought of something. Have the reaction part say "when you trash a card, you may reveal this. If you do, set that card aside, and put it in your discard pile at the end of the current player's turn." The only problem is, what happens with thief...
Not a new idea, and not the only problem.
One mechanical problem -- what happens if multiple people try to reveal the reaction? Who gets the trashed card then?
Balance problems -- well, it can get really, really swingy with trashing attacks, particularly Saboteur. There are already ways to steal Provinces (Possession + Amb/Masq; trashing attacks + Highway + Graverobber/Rogue) so maybe another way isn't so bad, but still -- it's something to think about.