Just a quick think-out-loud segment, here.
My current plan is to run mic/instrument into the Pisound, then split the stereo output into a direct mono out for live performance and the other mono out to the Reaper Pi; itself sporting a little usb interface to get the audio in. I have a JustBoom Amp/DAC HAT I could add to the Reaper Pi, to give me options of a decent headphone out for monitoring recording/playback tracks and some RCA outs and speaker connections some interesting use cases.
Over on another forum I am tracking a thread discussing the possibily of capturing audio on the Pisound Pi itself; if this turns out fruitful there may be a good argument for running a network cable between the two Pi's and having a setup that allows for dragging clips from the Pisound into Reaper. This network connection would also allow for the Reaper Pi to open up the Pisound virtual pedalboard interface for on the fly configuration.