Only i think this can be done is to use an application like MakeHuman or something that makes breast or SOFTBODY in the way its reacts only when its affected…
otherway for off my top of head theory would be use the FBX from UE$ or a rigg that works for maya or 3ds max and then add bones to maybe spine01 02? then remake a “PONYTAIL” but with the weighting of the breast etc softbody…
only other thing would be some sorta physics based on a mesh that would have to be hardcoded…
Hope you find solution would open up alot of things for UE4