If it's possible to transfer mods from a computer to a 360 (which, I know it is) and someone were to make another online co-op mod (like, or hopefully better than the last), wouldn't that make a type of two player mode accessable on Live?
I heard that Bethesda will realease some official mods that you can get through X-box Live. I don't know about your own mods that you make. But you couldn't make a mod then make it available for everyone with X-Box Live.