Due to liability issues, we don't rent our facilities to outside entities.
I'm not sure how adding the rental calendar would resolve admin issues. Unless your calendar program has a function to prevent double-booking it's nothing more than informational and still requires admin oversight. Outlook (not the web app) has a function to allocate meeting rooms which could fit your needs but I don't believe there is a function to incorporate it (as a feed) into dpcalendar (that's a global issue with freeware). There are a number of web-based calendar programs that can do what you want (I think calendar wiz is one) but, like outlook, they are not free.
I think that further explaining your goals and objectives (what is your optimal end-point) might help.