I had a crack at it and this is what I came up with: Route your system audio through soundflower as usual and for the track that you want to effect, just push the source audio to the system through the left channel only. You can route the right channel from Audulus (system) to get the effected audio back in your DAW.
The attached patch will take the audio and effect it according to whatever effect you replace my default with. It will then pass the audio back through the right audio channel.
I’m still getting to know my nodes so there are likely to be some quirks… as the usual precaution the attached patch has the volume for the return and the amount of wet signal turned down - just turn those up as you like - the waveform and meter output will compare the before and after so you can match your levels.
Mono Effect Mixer via Stereo Channel.audulus (98.3 KB)