I have 6509's that I connect to via one of the the supvervisors. If the supervisor that I use to connect to the switch with goes down I have to switch to the other supervisor to get in. Can I set up a loopback interface to use for a management address?
Yes if you want to although i'm not entirely clear i fully understand your requirements. Just make sure that each supervisor uses it's own loopback and that the loopbacks are advertised into your routing protocol and you should be fine.