Very neat idea! As long as it is warranty and finish friendly, I'm super curious how it works! Less valuable to me if it jeopardised either of those, but could still be perfect for you.
If you have any questions about it, please ask. We can invite whoever is most likely to have answers into the thread, if they're not here already.
I talked to plaster company and it sounds like flood effectively shot plaster warranty in the face. Basically in the event of a warranty call, they can point back at flood and as a violation of their water chemistry / quality clause. That being said - the acid wash guys from that company went full tilt on the pressure washers when they did their wash before fill... so the finish can certainly handle it. The water will likely slow down substantially having to travel through water from the sprayer tip to the wall - so I'm not too concerned in that respect. I hope it does the job though - agitating the remaining dirt off of the plaster...