"Certainly with full wing tanks you will get a lot venting through the hole, not so bad with fuselage tanks or partialy empty ones. "
Not on a pressurised refuelling system it won't. Self sealing couplings are the norm these days (pioneered on the Comet1 I believe).
Very rare to find a Boeing with a fuel cap actually fitted.
Airbus A320 on the other hand usually have it fitted as it is flush with the lower wing skin. If it is missing then as has been stated MEL allows for it with regular checks before each flight.
