I don't have IBS, I actually have UC (I've had it for almost 1 1/2 yrs). I've managed to hold down a full time position (37.5 hrs/wk)since being diagnosed. I teach exercise classes at a nursing home. I do seem to end up with more sick time than I did before having UC but I told my boss about my condition (after I was out of the 3 mos. probationary period). I mainly told her b/c I've been switched to a different drug that req. me to get my blood checked regularly so I have more dr's appts. now.
Bottom line, even when it's really tough you can't let IBS get the best of you. I think you should definitely try the FT position if that's what you really want to do. Good luck.