My board used to have this feature when we were using an old version of vBulletin. Unfortunately since we've upgraded to 3.6.0 the site owner/coder person hasn't had time to work on getting the hack back.
Once people paid the subscription the "Supporter" styles became available for them to pick from. We had a screenshot of each in the Gallery so people could preview the styles they couldn't use if they weren't Supporters.
Unfortunately I don't have the slightest idea of how to implement such a thing. So, this post probably wasn't very helpful. Except that I am very interested in this modification as well, if someone makes it.