Simple things, simple minds......

Guess it's be done before, took engine out ready for rebuild and had one of those moments, it's always been a pain in butt filling with oil due to location of filler plug, screwed filler plug out, tried a 1/2" bsp nipple in it I had laying about, perfect fit, short hose bung screwed into top of hose job done ?

Any reason it could,nt be took one stage further, take cam cover of, drill n tap 1/2" bsp for bung or weld a spigot on if required to do same job, thoughts ? Cam covers magnesium, weldable ?

Been done for years on lawn mowers!

Only thing I can think of is that won't you still need to be able to close it after filling? Can you reuse the filler plug?

as has been said
no problems what so ever
loads and loads of plant are fitted with this and a hydraulic hose into the sump bung and the other end taken to a bulkhead fitting for ease of draining oil.
believe me you dont want to lay under some of our gear to service it.
and you can get pipe blanks to seal the end no money at all.
as for welding Magnesium dont know where you would start...
cheers les g