With regards to Lloydie's suggestions, I think multipool.us already offers something approximating this arrangement - you mine, choosing what coin, then later you can convert it into BTC.

However, it's a massive leap to go from running a pool like this, to running an exchange for fiat. Any time you interact with fiat, by nature, you're playing it by their rules, and that means regulations and paperwork - most pools (especially the smaller ones of which Vertcoin has correctly encouraged a plurality so far) won't be interested in taking on, or capable of handling, the hassles associated. "Decentralizing exchange to fiat" is much easier said than done; you can't code your way out of this, because what's stopping it isn't a computer problem, it's a regulatory one.