What a nifty idea!
Taking that shape tool from MS Paint, basically, and putting it on top of a pumpkin... nifty indeed.
The only real problem is that if you want to carve anything NEAR the edge of something already carved, you're more likely than not to dip into the already carved area with the knife and end up "negative-carving," for lack of a better term, the gap. Like say, if you want to add teeth to the mouth you've already carved, you'll get little bits of pumpkin RE-added to the gaps if you go over even just a bit.
If there was a way to fix that slight bug, it would be the perfect pumpkin-carving sim. As it is, though, it's already among the best <20k flashes in existence, I'd wager. A great flash built upon a great idea!