Logically it needs a bigger hole than a single 1/8 diameter. It then needs to extend upwards a lot, so the gasses can go up while the oil has a chance to drain back. Then it would be even better if the gasses were directed into the air intake.

Crankcase pressure will force oil out of the hole.I'd think you need a hole,for ventilation of at least 1/2".
You can make a BMC A series type cap fit with a bit of fettling but make sure its the vented type.
Or just drill the tappet cover you have.
Or fit a vent in the metal cover plate on the dynamo housing (15mm plumbing elbow soldered in pointing upwards and tube to below the car if there is a lot of blow by).

I would take the Supaloy tappet chest cover off and replace it with a standard Austin steel cover that has the row of small vent holes in it. If this solves the problem of oil blowing past the filler cap, you can put your Supaloy cover back on, after first drilling some small holes in it like the Austin part has.
Alternatively, you could fit a union to take a vent pipe that discharges down by chassis level.
I have never needed an oil filler cap with more than a very small vent hole. Most of the oil that gets past my filler caps appears to be due to the fact that the caps are often a sloppy fit.
If you have excessive crankcase pressure it suggests that the pistons are not sealing as they should. This could be due to something simple like the mixture being too rich.

I had a problem with my 1932 RN throwing oil out of the small hole in the oil filler cap and around the cap itself. As a temporary measure I cut a disc of 1 3/4 in dia from sump gauze material to fit in the filler tube.
The oil hits that and drops back down the tube resulting in no leak whatsoever.

Location: Near M1 Motorway Jtn 28

Austin Reliant engines have a 45 degree slot cut into the filler tube at about 45 degrees halfway up, into this is welded a piece of flat sheet. The tube is not completely blocked as the insert only extends about 2/3 of the way across, but it acts as a baffle catching and returning most of the oil.
I concur with the other comments that a single 1/8 breather hole is insufficient.

Location: Auckland NZ

My three late 1920's coil motors all have the original conical gauze 'oil filler strainer' in the oil filler tube.

Not much use for catching other than quite large pieces but never had a problem with oil from the cap vent hole.

Standard Austin pressed metal valve spring cover.

Tony.

Location: Malvern, Victoria, Australia.