Well, its doable. All you'd really need is a 1:1 function equivalent for every OGL call. Problem is, so many DX functions rely on various Windows calls, you need to emulate those too to a certain extent. Plus, the overhead cost of running an interpreter.
Also remember its not just the graphics part of the game you need to rip out to make a port, you also need to remove all those windows specific API calls, and I can assure you theres a LOT of them. So making a OGL to DX wrapper won't magically make everything ported to linux.
Given the OS X port, its probably OK to assume those specific Win API calls are going to some sort of abstraction layer. They'll still need to write the functionality, but I imagine its a lot simpler when you're not playing hunt the platform specific code.