It shouldn't be that hard. I may look into this.
Really it's just install the game, copy the local files and apply the steam crack (for most games), maybe repack it into a nice packed executable with some lightweight compression of the files for easier storage.
I suppose the worst thing would be to manage the exceptions to the rule, games that use other drm, or that maybe need some tweaking for the steam crack to work,nor maybe need to create some extra folders to work. But for most simple games it should me enough.