Assign APM Remote Desktop resource from an iRule

I think what you are proposing is not that bad and probably a better option. You can use the "Advanced Resource Assign" object and add 9 conditionals with increasing RDP resource assignment.

We had a similar problem but we solved it a little differently. We let the user hit a page where all the PCs which she has access to is displayed. When the user selects the PC to RDP to, only that resource is assigned by setting a session variable to the IP of the selected PC.

Best.