I'm guessing, but I've yet to try this as I don't own an iPad, that you might be able to do the following.
1. Have separate user accounts on your Mac, each with their own instance of iTunes.
2. Begin with the iPad and User 1. Sync with iTunes and back it up.
3. Disconnect iPad.
4. Switch to User 2. Re-sync the iPad as a fresh/restore version, erasing User 1.
5. Repeat as needed when users want to switch.
Ideally, when someone is done using the iPad, they would sync to iTunes to back it up.
That does seem like a royal pain though. I'd head to the Apple feedback area and pipe in on the multiple user accounts request/issue for the iPad.
Again - I don't have an iPad, and can't say if the above will work. Just thought it might, as you can do something similar with iPhones (I did that for an old iPhone 2G that was handed down to my parents).