Saw a discussion about this on twatter so I'll mention it here:

There is a hilariously old PR on joinmarket-clientserver to add a feature to let takers split their change to match makers' change, when possible. You could think of it as babby's first multi denomination coinjoin.

One of the main reasons I created that PR is that the patch is isolated to *one* source file - "taker.py", and that makers already support this or any other way the taker wants to modify the coinjoin template; as long as they see their own outputs, they don't care about anything else.