Originally Posted by dinertime
By my calculations adding an ounce of 80 proof rum to 11 ounces of beer at 5% ABV will raise the alcohol to about 8% in the beer. That won't kill the yeast or arrest fermentation (think of all the bottle conditioned beers that are higher in alcohol than that).
Just to be safe I wouldn't do it to a whole batch the first time around, maybe just a six pack or so.
It won't kill the yeast, but it will bring your sugar-alcohol equilibrium far enough out of whack that the yeast will stay dormant even after adding priming sugar. It's not about absolute alcohol quantities, but about a ratio of alcohol and unfermented sugars. Big beers have higher alcohols, true, but they also have higher residual sugars.
At that quantity of booze, I think you'd be pretty hard pressed to get natural priming. I've not tried it, though. This is all speculation based on what I know about yeast attenuation, so I'll withdraw that answer if somebody with experience suggests I'm wrong.