Oil pulsing out of the hole is a symptom of the problems I described. Sealing the hole will give you a different problem.
There is a different source of slight oil drips in this area as both the Comet breather pipe and the chain oiler deliver to this area. The drips and dribbles that these may produce are very slight compared with the half pint pulses that a descending piston can blow out of an over full primary chaincase. I have only experienced the pulsing oil overflow once and it was completely cured by draining the crankcase and primary chaincase, curing the slumping and starting again with a correct chaincase oil level.
There is a different source of slight oil drips in this area as both the Comet breather pipe and the chain oiler deliver to this area. The drips and dribbles that these may produce are very slight compared with the half pint pulses that a descending piston can blow out of an over full primary chaincase. I have only experienced the pulsing oil overflow once and it was completely cured by draining the crankcase and primary chaincase, curing the slumping and starting again with a correct chaincase oil level.