I think a modified "silly-string" implant would be the holy grail. It was nearly the best expander of all time.
Imagine if you will, a expander implant, created in whatever shape you desire, with a colander like shell to allow water absorption, that is initially filled with silly string. As the string swells, the shell maintains the desired breast shape. There is a port for saline injection if growth is too slow that can also be used to maintain a particular size. The shell is attached to the ribs with titanium screws, assuring that the implant will never migrate. The girl could now choose between round, low hangers, or anything in between as far as shape. It does nothing to alter the weight, but the possibilities would be endless.