Oh, a new word for me to learn 
Sorry about the trouble. I have not yet tried to install SP on top of Patchbox OS.
But, you could use 2 RPis: one for Sonic Pi running on a previous version of the default system and another one running Patchbox OS with Pure Data. I honestly do think that for live audio (and all I plan) one RPi should only be loaded with one task (SP or Pure Data or Fluidsynth/Helm or animation aso.).
Nevertheless I do understand that such a ‘distributed solution’ might be a bit to complicated to control and you rather stick to one laptop doing all the stuff (though also there performance might quickly become a critical issue - especially with SP and in live context. See the recent Algorave where I spoted two or three systems being overloaded.