This topic could turn into a can of worms....but here's my 2 bobs worth.
The head of pressure in the oil tank is insufficient to assist any pressure to the crank even if all obstructions were removed. You'd need a lot more oil a lot higher up, imagine a 44 gallon drum balanced on top of your helmet.
100PSI forced into the inlet line would backfire at the pump and blow the inlet line off. The pump has meshed gears, unless these rotate the forced pressure fluid would revert to the path of least resistance, most likely backward. It won't go forward to pressure the crank.
What could be done for the cam lobes - a seperate feed injection into the front of the cylinder barrel to flood the followers, or cross drill through the end of the camshaft timing side bushing, then through the middle of the shaft and cross drill the lobes at the base circle. A feedline would have to be fitted into the timing side crankcase to bring oil up to the cam bush. In other words, a whole new timing side crankcase. Or even better, fit a cam that is actually hardened to a reasonable standard, i.e not Norton OEM.
40 PSI is fine at 4,000 RPM at operating temperature. 60 PSI peak is fine too, but anything over that and you've got your own personal Gulf Of Mexico potential. That's why the pressure relief valve is there.
Don't understand what you mean about pre-pressure to flush engine oil. You can't pre-pressure a dry sump system. Get the motor and oil hot. Drain the sump, and the tank. Drink a beer. Relax. After about 10 minutes, when there is no more drip drip out of the tank and sump, put screw the plugs back in and refill with your favourite brand of oil. Add a new filter too if that's needed.
Messing about with "engine flush magic in a can" stuff is highly suspect for a Norton dry sump system. Use good quality oil high in ZDDP (see muliple threads of Biblical proportions on this) fit a new filter every 2nd change and do it around every 2,000 miles as a guide or at least once a year.
Mick