Regarding cherries with pits in aging: Does anyone have a yes or no answer to wether the pits can produce a substantial amount of carcinogens?
I've heard that for peach/apricot pits. It may or may not be an old wives tale, but I'm not going to be the one that does the research! Plus - the whole peach doesn't fit through the barrel bung, anyway...
I've never heard carcinogens. they have cyanide precursors though (AKA Vitamin B something). And in exactly the right circumstances could produce the dangerous kinds of cyanide compounds if eaten in large amounts. I am NOT a doctor and do not have a definitive answer on this.
All fruits of the Prunus genus (Peach, Plum, Apricot, Cherry, Almond, etc) contain varying levels of amygdalin in all parts of the plant (including the stone), except for the edible portion of the fruit. Amygdalin gets broken down in your GI tract to several compounds, one of which is Hydrogen Cyanide. HCN is very nasty stuff. I'll never forget seeing my rat in tox lab, and hearing the dreaded words from my prof "I don't think he got the antidote quick enough". Which is a nice euphemism for "you killed him".
Now the amount is present in varying levels depending on the species. Apricots and bitter almonds are on the high side, while plums are on the low side, IIRC. Since I've heard that some of the pro's include the pits of cherries, I'm guessing that your exposure is likely to be below the threshold of toxicity at the amounts of cherries typically used in beer. But you're on your own with that. I sure as hell ain't trying it myself.