RE: white rifles
I have a 209 system for my Model 97 (thatI do not use)and I do think it would fit the G series White as they breech plugs can be changed out. BUT I am not sure if the hammer would work. The 209 does take a special hammer. Why not email White Rifles and see if they have a converter kit. Or if it can even be done. I was told they are back up and running with parts..
Personally I would just shoot the #11 cap. If you shoot loose powder, that #11 system is all you need. Just make sure to use a quality cap like the RWS, CCI magnum, or the Remington 40% hotter #11 cap. They all work on my Whites and I have shot them in all kinds of weather with little problems.
Also making sure the rifle is ready to shoot is very important. Swab that barrel, pop a couple #11 caps through the breech plug first, then load. After that, the rifle shoot fine. Also if you have a rifle where the #11 fits real tight, take some Emory paper and polish that nipple just a little down so the caps fit tight, but fit all the way onto the nipple snout. Actually a #11 cap is a lot more weather proof then you might think.