You have a right to decline if it's not in your written job description, you don't even need a reason. Naturally, you'll get the "not a team player" crap but there lies the meat for your argument.

In my experience, when at a company-supplied-beer party, virtually no one experiences difficulty in locating beer.

Maybe the answer is to suggest replacing yourself with a temp for the day. I hope you can work it out, that's beyond ridiculous to expect you to be a caterer in any situation.