It's likely not "spectrally neutral" enough to be a reliable gray reference. But there's nothing wrong with using it. And then depending on if your results are good, too light or too dark, you'll know how good a reference it is.
You are right there... there are far too many types of beer spilled on my camera case to guarantee any sort of spectral consistency.
Tom, on Point Pelee, Canada
Ansel Adams had the Zone System... I'm working on the points system. First I points it here, and then I points it there...