Something like that wouldn't bother the already existing models. Having a parented object to a bone is still common in animation nowadays with big pieces of armors and stuff. As for the animation, I don't know how the plugin would work but if it doesn't require to change the bones themselves to make it work, nothing should change. What the plugin would do (in an ideal world) is to make the users be able to rig skin over the already existing skeletons.