thanks daze for the link, glad to see someone else was thinking about trying it to.
i agree with you that vinegar is the last thing you should be using for your acid, but at the same time its probably one of the best. the reason why i dont think acid base reactions are the a good idea (with out proper filters) in general is because almost all acids are gases dissolved in water (one acceptation being acidic acid (vinegar)). that's why i was concerned about it being exothermic. as the CO2 is produced the solution gets hotter, as the solution gets hotter more of the dissolved gases that make up the acid are released and then redissolve when bubbling through your cider. even if it wasn't exothermic some acid molecules would still come out of solution, turn into gas, and go into the cider do to the natural equilibrium between the dissolved gas and undissolved gas.
the reason why i think acidic acid/vinegar would be one of the best choices is because it is naturally a liquid, not a gas dissolved in water, so you wouldn't get acid transfer to your cider. but you probably would get a vinegar taste to you cider, which is not good.
i think if i choose to try carbonate my cider using this method ill probably get my CO2 from a bottle filled with water, sugar, and yeast and pipe it over to the cider. then i dont have to worry about a vinegar taste or acid transfer. ill write a post to let you know how it turned out if i try it.