"The fuel line from the tank to the carb had sprung a leak. Clipped the hose and that was solved. The other leakage all appears to be coming from the valve cover...This has really got me stumped, because as far as I can tell, there shouldn't be anything more than crankcase pressure in there. The only thing I can think of is that, somehow, exhaust gasses are making it past the valve guides and into the cover? The oil looks fine, still clear, not burnt, not aerated or milky, but the stuff coming out of the valve cover, right where it is supposed to seat on top of the cylinder head, is almost foamy. When I reinstalled the o-ring into the valve cover, I used Hondabond as well, which didn't seem to help, as it leaked a little when I rode it today..."
I noticed in your photos that the rockers pivot in the valve cover. My Honda FT500 Ascot of many years ago had the same arrangement. I am not a mechanic and have no idea how common this rocker arrangement is with Honda or other engine makers, but I felt like it was problematic in my Ascot. He'll, my Ascot didn't even have your o-ring, just metal to metal. I am pretty sure all Ascots leaked from the valve cover. It is my understanding that no real valve cover gasket was used by the factory because the squish in the gasket would make tappet adjustments difficult if not impossible. This was made worse by owners, seeing the leaking valve cover, would try to tighten the cover bolts, stripping them out, so then you had fewer bolts holding the cover down. I read that Yamabond was the best for solving the valve cover leaking. Worked great for me.