Would it be safe to regard the prohibition against riding in a chariot (and thus an aeroplane which is even more opulent) as a 'minor rule' which, while appropriate for the Buddha's time, can now be dispensed with (and is
regularly dispensed with by even very strict monks, for the purposes of preaching the Dhamma)?
Having said that, a private
jet seems to be unnecessary when there are plenty of public ones available. It doesn't create a good impression,
seeing that picture. Maybe the rule needs some refashioning for the 21st Century, along the lines of, 'A Bhikkhu should choose, when possible, a less expensive mode of transportation, if such an option exists; and such travel in a vehicle should have a purpose that is in accord with the Dhamma'
(so not only preaching, but visiting a sick parent who is overseas would be in line with Dhamma, one would think).