That is a great idea! I really like that, make it so you can donate if you want to, and leave the amount up to the user. The law of averages says it should work out to be the same in the long run 
The smarter option, which I mentioned, is creating a charity pool or using ASICs to mine these charity coins. Both help secure the blockchain and add to the stability of the coin without taxing the average user.
-Fuse