It doesn't need an extension to the API, you just
have to be able to make
input -> input connections. I belive that makes more sense in an abstract
sense (eqivalent to wiring it that way in hardware) and internaly (reduced
complexity).
This will work if the powers that be agree to allow input -> input
connections. If not, there is another way. In a hardware mixer,
many-to-one connections always require a bus, which is nothing more than
a unity gain summing node. Since jack performs unity gain summing for a
many-to-one connection, then the buffer that the sum is written to could
be redefined as a bus. Then any client could read from the output port
of this bus. I am not necessarily in favor of this, nor am I against
it. I don't know enough about coding to have an opinion either way.
What I do know is that this approach preserves the output -> input
semantics.
Tom