It's in the administrative code:
http://apps.leg.wa.gov/wac/default.aspx?cite=314-11-050

The WAC consists of the rules government agencies make, based on the regulatory authority given to them by the legislature in the RCW.

So the liquor board itself would need to change the rule. But an act of the legislature or an initiative could force them to do so. Personally, I think strip clubs gross out a sufficient number of otherwise liberal-minded voters that, combined with the more traditional moralists, there's not much chance of an initiative or legislative solution. The best bet is probably appealing to the board directly. Although this is still pretty unlikely, as an unelected and ostensibly non-partisan body they are perhaps in more of a position to listen to reason. You might need to come up with a better public policy reason than "I wanna see genitalia and drink beer" though, even as compelling as you and I might find it.

The issue with the court case in Maryland was that the law had an immensely vague definition of "adult entertainment" that didn't have anything to do with nudity. The decision wasn't that nudity in the strip clubs was protected by the 1st amendment, but that other things that ARE protected could be caught in the law's vague definition (among other things, it was speculated the Baltimore Ravens could be so labeled because of the butt-patting). Unfortunately, this probably has no bearing on the situation here, where the rule is very specific and goes into great anatomical detail.