Did you just leave these at room temp? Or did you try an elevated temperature? In the literature on astringency removal, a hot water bath at warmer temperatures takes hours instead of days. I think with the hybrids and Americans, that will be crucial to winning the race against the normal softening process.
Also, one thing that the researchers do (and I believe commercial operations) is spritz the fruits evenly with the ethanol source before sealing them up. Apparently, the amount of ethanol needed is only a few mL worth.
I’m still twiddling my thumbs waiting for my trees to come into bearing, but I can’t wait to actually test this out.