Great! I think its the sort of thing that needs to be built and demo'ed before people understand how terribly useful it is.
I wanted to clarify what I meant by dividing up the jacks into a group of 3 buffered mixers. As you know the current multiple setup is passive and consists of 4 jacks. Any of the four jacks can be an input and the remainder can be an output. Of course, you can also use passive multiples for crude mixing, but you don't necessarily have definite control over that.
So with 3 buffered (and cascading) mixers you can think of all the 'a' jacks as your input, and the b, c & d being your outputs. So if you plug into 1a then that signal is buffered and cascaded from 1b - 3b, 1c - 3c & 1d - 3d. In that case, its a 1:9. Now if you plug another input into 2a, then you interrupt the cascade and now you have two groups: 1a input with 3 outputs 1b, 1c and 1d : 2b input with 6 outputs 2b, 2c, 2d, 3b, 3c and 3d. So if you plug another input into 3a, then you interrupt the cascade again in the same way.