Find a place that does inflatible repairs and ask them for professional advice. It might just be worth the money to have them do the repair. I don't know if The Inflatible Boat Works is still in Ballard (near Seattle) but you can probably find some place on the eastside. Maybe contact some of the river raft guides as they probably have to do occasional repairs.
I have done small repairs (cuts and pin holes) on my old one but not over large holes. If you make a patch that overlaps at least two inches around, I would think it would be fine. Be sure to clean it and do all of the prep work meticulously. It would probably help if you can find/make a curved surface to match the shape where the hole is, that way the patch will fit better and lay flatter when you glue it up. Be sure to take the other side of the tube into consideration when shaping. Another thought is that with that large of a hole, you could put an interior patch in first and then another on the outside. This would definitely give you the strength you would need.
