Not a bad idea.
To keep it manageable (and perhaps more fair) you might want to limit it to one submission per member.
As for judging, having all board members vote on the first submissions might be intractable, or simply ill advised. If the submissions come from the board members, and the votes come from the board members, you may see the votes spread pretty thin, as people will tend to vote for their own work. You'd prefer the winner to win by a large consensus, rather than by just a few votes....
For the first round, you might want to do as you did for the ENnies - have the boards elect a panel of judges. After they've narrowed it down to 10 or 15 proposals, you can then let the full membership vote.
What charity? Well, the simplest would be to allow the winner to choose. If you prefer, you can either desgnate one yourself, or again hold a poll to choose an appropriate charity.