Couldn't the Mono or Stereo Loop, with mix set appropriately, be used in this way already? The only hitch is that you have additional FX post-loop, and therefore shared by both rigs
I was thinking of using the (mono) loop for adding external effects to the KPA rig, while using the Alternative Input to introduce an external second rig.
Thus, two rigs in parallel.
I can do this now using a mixer. This would just be easier.