I wonder if the blocked oil feed pipe might be a clue? Could this have been blocked for some time, perhaps, leading to high wear on the rockers and shafts to the extent that they now leak lots of oil? Running the engine with the rocker cover off might prove illuminating here.
The rockers and shafts are quite sloppy. The replacement I got for the broken front rocker shaft (kindly donated by a club member) was quite pitted but works perfectly adequately. I have run the engine without the rockerbox lid and I do get a fair amount of oil splash from the rockers. This was with a cold engine and I suspect that it would increase as the oil warmed up.
Is the pressure relief valve working properly? My bike runs at a pretty constant pressure between 20 and 30 psi once it has gone through the initial warming up. If you are getting a higher pressure than that consistently, suspect the pressure relief valve. If this isn't working, the excess oil might well be coming out via the rockers.
When I first ran the engine the oil pressure was quite erratic so I fitted a new ball and spring to the relief valve and set the pressure at 45psi as per Pitman book. With a cold engine the pressure remains steady at 45psi from tickover to full speed. When the engine is fully warmed-up the pressure drops to 20-25psi at tickover but rises back up when riding and stays around 40-50psi.
You have got the oil feeds the right way round haven't you? The small diameter pipe feeds the rockers, the large diameter one feeds the oil gauge.
Yes - narrow bore pipe to rockers.
You are probably correct that more oil than normal is getting to the rockerbox but, apart from the leakage from the breather, is this necessarily a problem? The excess oil drains down to the cam followers and that must be better for the camshaft. I am still considering a simple expedient of running a hose from the breather hole to direct any oil away from the cylinder head - maybe tee-ing into the oiltank breather hose. I also don't think excess oil is going down the valve guides since the plugs appear to be quite clean.
Interestingly the 1939 basket case 4G engine I got last year had a small brass tap fitted in the rocker feed pipe. Maybe this was an attempt to remedy a similar issue?