I think it does not matter how you position it, the airline baggage handlers will find a way to do it differently. And, the plug should not leak, so not sure how that could affect anything.
Just yesterday I wrote up a short piece on my thoughts on air travel and my Rohloff hub and posted a photo in a different thread. To summarize, I changed my oil before I took a trip in 2016 by air. I suspect that the wheel was shipped with the left side down and my EX box became very oily and dirty over the next several weeks after that flight, see photo. I suspect that a lot of my new oil came out of the hub from air pressure during the flight.
But last year I did not have any such leakage when riding the bike around home after another oil change. Thus, I think that leakage was all a one time event from the airplane pressure variation during flight.
I think you should just bring some extra disposable paper toweling along on the trip in case you want to wipe things off if they get oily. And maybe some disposable gloves like medical personnel use to clean things up if necessary.
Next time I travel by air, I will plan to change the oil after I get home instead of before the start of my trip.
