Can't you just make a server-side mod that has multiple redirect entries and it automatically checks each one for the file if it doesn't find it? (Like Demo manager does).
Forgive me if that's exactly what you are describing, the PHP thing throws me off, not all that keen on that stuff
Would be a very handy thing to have though. I always wondered why there are so many random mods nobody will ever use out there, and obvious useful things like this were never made