I know I can get a list of PO users via the getUserListRequest call.

Is there a way to get a list of all users system-wide?

The Object API has a call on an Account object that allows you to retrieve the system address book (and if you are authenticated as a trusted app, to specify that you want to see ALL users, even those with visibility set to None, or PO, etc.). Is there an analogue to this functionality in the GW Web Services API?