Wouldn't that be loverly!
I don't know anything about code-writing and such, but it just makes sense that if co-op MP wasn't put in to begin with, it would be extremely difficult to add, in any satisfying way.
From a forum about Oblivion, I heard about Hellgate-London, and for a couple of months now, my wife & I have enjoyed playing that game, as a co-op team. It took us a while to sort out things like the Microsoft headsets, (with the gushey David Clark replacement ear-pads), and the Xfire in-game voice-chat, but, all things considered, Flagship Studios did a great job providing entertainment for an elderly couple. It is too bad that Bethesda didn't/wouldn't/couldn't, make that happen. Maybe next time.
Meanwhile, we're always on the lookout for games that fill the bill.
(Where else can seventy-something pairs of vanquishers-of-evil-demons do their work, as if they were still twenty-somethings)?