The problem with this that I can foresee is that the breast is a physical object that drives child nodes, and you're proposing reversing that relationship, and that might be difficult.

Right now, when a character grabs a breast, the physical simulation of the breast takes precedence, and then the hand follows the movement of the breast.

Just contemplating possible technical solutions to this:

1) Permit toggling a physical object like a breast between physical mode and IK mode. The downside of this is that when in IK mode, the physical object ceases to be physical at all, and would need to be manually animated to prevent undesirable results.

2) Enable users to create and animate arbitrary forces applied to physical objects like breasts.

maybe have a target node similar to the look at target node that has a weight influence slider? have the nodes hidden with 0 influence and parented to the spine by default, then have the bones target the node / use the same physics as mouse grabbing them when influence is set to 1 so that it could be stretched hard?

actually after thinking about it a "grab node" would just be best, have it grab whatever local surface it snaps to and it can re-use all the code the alt click grab uses. that way it wont just be limited to breasts but all physics objects

anything to make it possible. This game has such amazing boobs and physics that it is a waste not being able to play with them