I am thrilled to say that I am not a litigator!
When I first started my career, I did do litigation on the civil defense side (defended large companies in product liability actions). I hated it. I wasn't diagnosed with UC at that point, though, so the courtroom wasn't an issue.
Now, I do corporate work... all transactional. So, the courtroom issue is not one that I have to deal with. That being said, I have had to excuse myself from client meetings, etc. I just simply say "excuse me, I will be back in a moment" or something to that effect, and I step out and do what I need to do. I've never had any repercussions from that. Of course, I don't go mid-sentence... I try to wait for a natural break.
As far as suggestions for your situation, here are my thoughts:
(1) Remember that the judge doesn't care about
your personal issues. I wouldn't recommend disclosing your Crohn's to him or her. I just don't think it will get you anywhere.
(2) As far as asking for a recess, I have seen lawyers ask the judge for a 5 minute break as they are not feeling well. I've never seen a judge argue with that. I once tried a case with opposing counsel who had the stomach flu. He was in and out all day long.
(3) One point I will make is that a lot of this depends upon the judge you are in front of. Remember that they don't have to grant you a break and some may not (it goes to that whole power thing - it's their courtroom and they will run it as they see fit). They also have a schedule to keep to on their docket for the day. I think the best approach is for you to disclose your "situation" to your colleagues so that if you do have to step out, they can pick up the pace until you get back. That way, no one is disrupted and you don't have to take any flack from a grumpy judge.
(4) Finally, I find that when I am speaking in front of a group or concentrating in a meeting or otherwise involved in a situation like that, I rarely experience any urgency. It must go to the whole mind/gut connection theory, but you may find that you might not have that issue as much as you suspect. Of course, nerves always play a factor depending upon the judge!
Good luck to you in the rest of your law school career! I hope you find that you enjoy the profession.