Bill:
I currently run Exchange 5.5 at my work location, and have not run into the problem you are having. However, I am sure that many might agree that there just may be a slight leadway into the problem you are having, by not being up to date on Service Packs. You might find that updating Exchange 5.5 to Service Pack level 4 might help you out.
If your Exchange Admin console is locking up when trying to display hidden recepients, it is safe to assume that this is a display/refresh problem. It also just have to do with the properties that it is trying to bring up on that custom recepient that you created with any other hidden recepients that exist. Attempt to display hidden recepients at least 3 times in a row when going inot theExchange Admin console, if it hangs/locks on all attempts, try the Service Pack upgrade and see if you see any improvement. It is a good idea to get up to date on Service PAcks none the less. Good Luck!
Paul M. Chavez