Yep, they make fiber optic sights the clamp right to the rib. They work pretty nice. Mostly used for turkey hunting but you can use them for slugs as well. I did it with my Mossberg 500 and was pretty happy with them. Get some decent ones though, the cheap ones are pretty chintzy.
Cabela's sights page
I actually got a set at Walmart once. I would bet Gander Mountian, Bass Pro or even your local gun store would carry some. The ones I got at walmart came as a package deal with a turkey choke as well. They were kind of cheesy though, the other set I got was around 40 bucks and they were a bit nicer.
Paul