Could it be possible to save a note to yourself? So that if you play a card, and your opponent is offline, you could write in a little box "YOU NEED MORE WALL!", and when you open that game and it's your turn, a little box is there, that you gotta click "OK" on.
I mean, I've lost, without a doubt, 50 games, where I thought "ok, next turn I've got to play Barons keep, so he can't just kill my tower or wall", he's offline, and like 20 hours later, I go into that game, and play another card.
So just a little button that's called "Note to self", you click it, a box appears, you type in a short message, click OK. When you join, it pops up, you gotta OK it, it disappears, back to normal.
It's really not cheating, since it's easy to make mistakes when you've got 13 games running at the same time, some guys playing 20 seconds after you played, some 20 hours after you played.