I don't think beer will "damage" wool at all. If it were me, I'd just rinse the affected area out thoroughly, let it try, and be done with it. But that would be for random minor spills.

If I were going to work a beer stand where I might have repeated and ongoing risk of getting beer on my kilt, I'd probably just don a canvas kilt apron for that stint at the beer stand. That way, when I was done, I could just remove it and go on with my day without any further worry. They sell WWII surplus ones at WhatPriceGlory for $18, although I'm sure there are numerous other places you can find them as well. In a pinch, even a regular household apron (or even a more manly tool apron from the tool department) would work.