jbhiller,
Interesting idea...the rental.
I own one and find I don't need to use it most of the time. With my ET arm, I like using it as this arm has to be manually twisted on it's mount to adjust azimuth. This isn't a very accurately repeatable feat so the Foz really helps. On my VPI 3D arm or my Townshend/Funk Firm set-up, I don't need it.
In my (symmetrical) room, I've found if I have my speakers set up identically, I can adjust azimuth by ear very easily on the last two arms I listed.
May I ask why do you feel you need one?
Cheers!