I think you might be able to set up each project by logging on to the operating system as a different user (which would give them different preferences files) and then use fast user switching to login to a different ‘user space’ for each project.
I haven’t tried this, but it should work.
From Mac Help:
You can turn on “fast user switching” to allow more than one user to stay logged in to a computer at a time. For example, if you’re working on a spreadsheet of financial data and your daughter begs to check for iChat AV messages, you can switch to your daughter’s account. When she’s done checking for messages, you can switch back to your account and your spreadsheet is right where you left it.
When fast user switching is turned on, the name of the current user is displayed in the upper-right corner of the menu bar. You can click the name to select another user to switch to. If the other user’s account doesn’t require a password, the computer switches automatically to the new user’s home folder. Otherwise, you see a dialog asking for the user’s password.