Situps/crunches are crap and are also bad for your back. Planks are very good at building core muscles and lower back muscles (also other muscle groups but that isn't the focus). I don't see why they would be bad for ostomates. I do them all the time. And there are lots of variations.
You could also use an ab wheel.
If you really want to build your core do weighted squats. Lots.
Edit: Forgot deadlifts as well.
Post Edited (NewShinyCD) : 3/19/2012 8:06:24 AM (GMT-6)