I of course have no idea on how to actually do it but I am in the process of putting a motor on the knee so I can be ready.
When I have the knee drive all set up, I intend to just go into the G-code and
Input the values manually. If you know the difference in tool lengths, it should just be a matter of inputting a C (knee) move just after the tool change.
If somebody else has a different take on this, please speak up.