Its common to block off the drip feed to the chain and lubricate the chain manually,
this drip feed to the chain from the engine return is new to me, are you thinking perhaps of the tanks breather muwump, I know this is sometimes used to vapour the chain , I like you use a spray.
Don't think there should be any restriction at that point Tim, too much oil to the rockers is a perceived problem, check the holes in the studs that feed the rocker spindles, A's use a very small hole, other BSAs (Goldies I think, could be wrong help me lads) use studs with a bigger hole, this leads to over oiling of the rockers