Originally Posted by david_42
I can't think of a worse use of beer. I'd rate using it for snail bait higher. All that will carry over is the bitterness, unfermentable sugars, and the color. Hop flavor, aroma and alcohol will boil off. Unfermentable sugars will further caramelize. Utilization and subsequent IBUs will be impossible to calculate. Instead of having two batches of drinkable beer, you'll have some horrible sludge.
But, it's your time, your money, and your beer. Have fun.
You know this from personal experience? I agree with the OP that it could possibly be used to re-purpose a beer that might otherwise get dumped. Further caremelization of unfermentable sugars might provide a more interesting flavor profile. Assuming the batch size of the original beer and the beer being brewed were the same, bitterness could easily be calculated by adding on the IBU from the original beer (hop bitterness doesn't get boiled off) to that of the new beer with utilization being a function of boil gravity. You'd have to do it on the fly after you take your boil gravity reading, but it could be done.
I agree with your point that the hop flavor, hop aroma and alcohol will get boiled off, but if a brewer chose to do something like this, they would be accepting these as justifiable casualties.
Also, I just want to make it clear that I am not saying you are wrong, I just don't think you are necessarily right, either. I'll be interested to see how this turns out if the OP (or anyone else for that matter) gives it a shot.