Hmm... I just remembered something which may be useful in your quest for real time jiggling. Fire attronachs have a bone in them which lags behind the movement of the fire attronach in real time. Fire is attached to this bone, so it leaves a trail. You could probably check out the settings on this bone and apply something similar to the breastbones. (I'm 90% sure this was done in the bones system, not the NIF. Throttlekitty might know more). It wouldn't have a perfect result, and it wouldn't take momentum into account so no actual jiggling would occur. It would result in basic secondary motion though. The lag would obviously need to be set to a fraction of a second, otherwise the breasts would end up about a meter away from the body when you run. Even if it isn't an acceptable solution at least it proves that there is room for experimentation in the current skyrim engine. EDIT: I've checked out the fire attronach. I think some of the info is in the .NIF and some seems to point to an external .hkx file. Theres a property called BSLagBoneController with various settings on it. I'm not sure if adding this on it's own would be enough.