Hey sdusso,
Using the restroom during the test is permitted, but not advisable. Here's the applicable LSAC regulation, from their website:
If you find it necessary to leave the room during the test, raise your hand and obtain permission from the supervisor. Your test materials will be collected and held until your return. You will not be permitted to make up the time. Use the restroom before reporting to the check-in station; once checked in, you will not be permitted to leave until after the start of Section I.
So, you are free to go after the start of Section I, but ideally you should wait until the 15-min break between Sections 3 and 4. If you anticipate this being a huge problem, there is always this:
http://www.depend.com/
No one will know, and it's not on the list of prohibited items.
Hope this helps