If you can get to the underside without removing the coroplast (if present) and a lot of other work, an offset flange would work. You *might* be able to do it from above if you're careful and make a clean cut in the pipe into the tank. I'd use a die grinder with saw blade on it, but if you don't have one, a dremel tool may work but would be slow. A flex shaft on a Dremel may help.
Another solution would be to create a recess in the wall the thickness of whatever the studs are - 1-1/2" or 2-1/2" if the wall has a vent pipe in it. Then trim it out to make it look nice. If the vent pipe happens to be behind the toilet, this wouldn't be possible.
