Apologies, you're quite right. I thought I'd maybe missed a drilling but no, there's no other place for the pressure release to go than that oilway up to the camshaft trough.
A longer session today suggests that my (1955) cases aren't completely blocked, just very slow to get to/from the release valve end. Might just possibly be the way I'm trickling the paraffin in, I'll be giving it some more attention as checking/cleaning the crankcase oilways is the joint next thing on the 'todo' list (tied with putting the rods on the crank only that could be going through a prolonged pause - I hope you've got a big-bearing crank in that engine or you might have some problems with the big-end journal sizes...).
I've got another set of 1955 cases as well - very much abused but they seem to pass straight through that oilway despite being filthy as well as battered.
(I've seen many more used/clapped cams and followers than used/good for these engines so that could be an achilles heel even with a squirt from the pressure relief system - I'm hoping that modern replacements and a sympathetic servicing regime add longevity!)
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)