Using ES2 which is supposed to be a 5.1 instrument here's what I've done so far.
In I/O assignments set L+R to 3+4. Set Ls+Rs to 5+6.
Create Aux (bus). Chose as inputs 3+4. Create Aux (bus). Chose as inputs 5+6.
I'm getting "something" this way though I can't really verify that it's what you need as I don't have any instrument nor compressor that works of quadro. Logic has always had it's own internal sidechain so none of this has really been necessary.
it's the same as reaper, and even reaper AU is working. You have 4 channels, first 2 are your audio, the other 2 are the sidechain control signal.
In vst world you just add a quadro channel and that's all (quadro is a speaker configuration type coming from specifications). Cubendo loves it.
In au world there are 2 possible solutions. The first one is the actual one, you have a 4 ins - 2 outs on a single bus. It works more or less like the vst version, reaper works flawlessly.
A better solution would be creating 2 stereo busses (they call them "elements"), each one with 2 ins and 2 outs. Anyway I have troubles calling the rendering function, I asked on kvr audio forum too. The worst case it that logic users will wait till I find a solution.
I've done what you describe and logic just sends everything thru the plugin and out again. The sidechain signal gets send to the master channel and doesn't really seem to affect the none-sidechain signal.
I fear that you may need to figure out how logics internal sidechain works. I see it being used by Stillwell, Ddmf, Psp, Voxengo etc so maybe it's not too complicated.