I have done a 10V with an O ring, that has run quite a lot on display with no problems. Groove size was as per the Tubal Cain Model Engineers handbook. I also used O rings for the gland packings. They are made to look like a conventional packing with two studs but there is a concealed shoulder so the O ring has the right side clearance. I can get the engine to run by blowing very hard into the inlet.
I've also done a Double 10 with actual piston rings. They have to be pretty small in section, but I followed the Chaddock proportions and they are fine…they spring onto the piston without breaking, there is little friction and they appear to be sealing fine. My Stuart Triple also has cast iron piston rings, but is not far enough along to test yet.
John