As anyone ever tried to backsweeten a batch of cider, after racking into a keg, with AJ that has preservatives in it? Would this also help in keeping the yeast from fermenting any further, in the keg?
Preservatives don't stop fermentation. It stops the reproduction of the yeast. It would still ferment if you had a large enough colony to eat all the sugar without reproducing. If you put the keg in the fridge and keep it cold it wouldn't ferment either.
As anyone ever tried to backsweeten a batch of cider, after racking into a keg, with AJ that has preservatives in it? Would this also help in keeping the yeast from fermenting any further, in the keg?
Folks who have successfully fermented juice with preservatives mostly don't even report a lag time after pitching. Preservatives are meant to prevent the spread of random small cultures of weak wild yeast and bacterium. They're barely a speed bump when it comes to modern commercial strains.