I'm with you here. Have 99,999 be the maximum. That's how the latest games do it, and given the choice, I'd rather go with what the newer games say than stick to the letter of an old mini-game.
I was going to suggest having a Setting which allows the user to easily define the coin maximum, but I think if users change that, it'll cause problems with the graphics of the mini-games (i.e. not enough digits being shown). They probably wouldn't touch it anyway, but just in case they are tempted to, I think it's best to leave it out of the way in the mini-game scripts themselves.