I would bring a half-used bottle of whatever crap liquor I have in the back of my cabinet. It might have bugs in it. Because that's how long it's been since I drank that crappy liqour.
I would bring a case of water. Because if people are tacky enough to throw a party to get alcohol for the wedding, god knows they probably won't even provide aqua.
My initial thought when I read OP was ... will they be serving beverages at this "stock the bar" party? and if so - isn't it kind of counterproductive? Why wouldn't you save the money you spent to throw said party and just use that towards your wedding booze??
Logic... it's so hard! *rolls eyes*
ETA: because typing is hard too
I know people who did this for their wedding...which was also potluck...they are well-meaning people. Maybe I don't hold it against them since I didn't know them then, but it did make me double-take when they told me!
Re: "Stock the bar" party