Over the years driving  and the miles, closing in on 2 million ! [over a million across 3 XJ Cherokees] I've tried most of the options.
I guess it comes down to I don't want to spend the entire day changing diff fluids across 3 vehicles if there's a quicker way.
I like the cover drain plug idea best -- if I could weld thin metal like the covers I would but my welding skills are limited to plate steel -Think JB Weld would hold the nut instead
Drilling and tapping the diff housing has occured to me was always worried about strength and internal clearances - how many others have tried that"
Once the cover is off I agree an RTV sealant is the best bet but we're back to that time thing again -6 Diffs = a long day that way
Rubber reusable gasket would be good -- cork leaks for me as often as the RTV
Will look into the transfer pumps for 5 gal containers -- have tried a bunch of smaller ones overthe years --all worked - just slower than I like
Any boat people here? how is the engine oil changed in an inboard speedboat engine ? They're not doing it from below
and certainly not pulling the engines for an oil change
I figure there has to be a decent elec pump the boaters are keeping a secret !