Tried my fix this morning. I opened a beer, dropped in the priming tabs and had to dodge the spew. The tabs provided condensation nuclei and started a mad rush of carbon dioxide from the beer. So second lesson learned, don’t drop a priming tab into a carbonated beer….
So I’ll let the beer go for now and see if it picks up any more gas over time. And if I really decide to fix it, I can dissolve sugar in sterile water, add a measured amount and let the beer gas up. Seems more trouble than it’s worth for a half-batch of beer that’s pretty good even a little flat.