I use a bespoke printer manager for Citrix which does a similar thing.

I have two windows - the window on the left shows the users currently connected printers, the window on the right shows printers available on the print server. Double-clicking moves the printer from one side to the other. There is also a "set default printer" button.

It's a large script which is heavily biased to my environment, but you are welcome to look at it to get some ideas.

I'll stress up front though, if you have problems with it I will probably not have a lot of time to help as things are about to get very busy round here...