We also played with the general rule that you can do anything outside the rules and as long as you can get through your turn without anyone calling you out in it you're in the clear. Playing more cards than you call is a pretty common cheat and it's always pure chaos when it happens and gets called out.
It can be a real friendship stressor so you have to have the right group of players for it, but it's always fun sharing the stupid cheats that people pulled off afterwards.