Tom. Since you revived this thread, how did it work out for you?
It worked well. I took 2 approaches to fixing 2 beers with opposite problems.
First was a under attenuated Festbier that finished too high. I boiled 3/4 oz magnum (opened in freezer for a while in a purged bag), and 1/2 oz hallertau (same) for an hour, ran it through a french press and dumped in the keg, allong with some gelatin. Totally made that sweet beer palatable, not perfect, but I could drink it and be happy.
The second was an IPA that way over attenuated (got too hot) and finished at 1.005 (like 8% abv). This beer also fell down the stairs with me as I tried to take it to a cooler part of the house. It got really shaken up, air lock broke off, and got oxidized a bit. Lost almost all of the hop character, just had an apricotty taste and aroma from the heady yeast.
For this beer I didnt want to add bitterness, but aroma and taste. Boiled a half oz of citra for 5 min, and added another half oz at 180 deg in the french press, and did a hop stand for 45 min. Same deal added to the keg with gelatin. This beer gained some flavor and some aroma, but not enough.
Also FWIW, each keg was down to about 3-4 gallons left for these measurements. I wish I had done more in the IPA.
So I am glad I did it, and I am enjoying my festbier, the IPA will still be a drain pour when my next beer is ready for the keg. Too thin and estery.