My main idea is that you use monocontraptions for a multichannel cable.
To make a multichannel cable you have contraptions that look like mixers but their output isn't mixed. They can link together with for example an 8x output drums contraption.
You can split them off through an contraption, that has one input(multichannel) and addapts its amount of outputs to its input.
Sidechains can use a contraption that works like the splitting ones. Only that you can decide which channels build which outputs. They have an inside like containers(for example the one input leads to 8 inner channel "inputs" 8 inner "outputs" but to as much outer outputs as inner outputs are connected. The main routing, that goes through, is represented as an 9th inner input and output) If you don't want to keep the whole 8 channels in the main routing, you had to connect the channels from the 8 ones not from the main 9th one.
A trick is that you can use the shift key(or caps key) in the middle of a cable to create such a contraption.
Maybe such contraption can dock to a stereo delay?! (with a circle delay option).