I had the same problem in college. This is what I would do- I regulated my diet to where I knew if I had a "safe" chance, I could go late afternoon. Then, the campus library was next to my dorm, and they had basement bathrooms. No one was ever down there. I would just go use that bathroom and would be fine until the next day. Worked like a charm.

I have done the setting the alarm for the middle of the night. It is not fun, but it works.

Now that I am out of college, I still have my phobia, but I have come up with a way to tell people about my problem without coming out and saying it. I buy digestive cereals and talk about eating raw green veggies and nuts to "be more regular". The more I talk about it through food, the easier it is becoming to accept going.