New Request - Workstation Selection

I've changed the request default value so that only workstations associated with a user will be displayed for them but I've noticed a small problem.

I have a handful of roaming users who have no associated machine (they use what's available). If they have a problem and open the New Request form there is no workstations available for selection (as it should be).

Is there anyway to have the form display only those workstations assocated to users (who have one associated with them) and allow non assocaited users to access the complete list of workstations? I know admin can do this so feel that there may be a way around this...or perhaps it could be added as a possible feature if it's not possible?