I think it's a bit of a stretch to think that Eno and Lanois wouldn't produce the tracks they have co-written.
Personally, I think it's all but certain they will at the very least co-produce the record.
Rubin likes to work with songs that have already been written, so he certainly could spend a few months withe band after they've written all the songs, regardless of how many projects he has. Metallica has been writing that album for almost two years, off and on and I'm not sure Rubin has spent more than a few weeks with them. He's supposedly waiting until they are ready to record.