Someone call me stupid. What would be the problem with a screw-adjusted chainstay-mounted tensioner? It seems to me like that would allow you to quickly and easily turn any junker into a fixie (provided you have a wheel) for less than $10 in parts without worrying about chain tension/length or finding a magic ratio. Plus when the rusty chain gets longer, you just tighten it up.
Obviously this is a solution looking for a problem on pricier bikes, but for the dumpster bikes you take barhopping, it seems like a good idea.