Hi there Nathan. I dont think using bags is appropriate, this is because 90%
of effective atemi is based on angle, and hitting the correct point of the body, with the correct power.
i.e. if I was about to apply shihonage I wouldnt give him a (thousand cracking fist of the north star) punch between the eyes, as it would knock him over probably and it would ruin the movement of the throw. Instead a stern strike between the eyes would be more appropriate. Maybe there in not bag use as we dont really need striking endurance, which bag work develops. Also I feel most of the offencive atemi my style uses in aikido couldnt really me done on a bag. For instance, one tecnique is before uke launches the attack, tory slices his arm across and up uke's clavicle using hip power etc sending uke to the floor (god its difficult to explain tecniques in words)
that is really difficult to do on a bag.
Although bags are good fun to play with.