I think I already know the answer to this, as I haven't been able to find any mention of this anywhere in the documentation or forums, but is there a way to track the cumulative amount of time a Premiere project is open for billing purposes? Is there project metadata hidden anywhere? Maybe similar to the time data used in PPBM5?

Hi Adam. Something like this might be of interest?
Try DesignSoft StopWatch Plus, Time Sheet Manager, and Creative Billing Online! (http://www.designsoft.com/)
through there seem to be a few stopwatch type programs out there.

Do a "Save as Copy" immediately when you start working, when you take a break, when you return from break and right before you finish working on the project for the day. All these files will contain the exact timing in the file properties and you can easily calculate your working hours from them.