papajam· Trained (ex)Professional, , 1953-2018 RIP
Wouldn't do a bit of good. The problem is not water intrusion into the cap but condensate inside the cap....maybe we should kinda waterproof it with , I dunno, duct tape? A deflector on the base of the dist?
Warm air condenses on a cooler surface (basic physics). So you have warm air in the cap that has now been chilled by water. Result? Condensate forms inside the cap. One could spray the cap on a cold engine with a garden hose and the car will start right up. Now spray the cap with a hot engine. Possible no start.
The two piece plastic distributor covers mentioned by Robert were standard issue on all L4 Alfa engines from at least the 1750 on. But they greatly hindered servicing the distributor so most were never put back on.
Oh, the holes in some caps are air vents, not drains.