Often when you see people on TV doing "around the world" stuff, they actually buy separate motorcycles for each continent, often moving accessories and such over to the next bike. Given the logistical costs for such endevours, the extra bikes are not a big deal and bypass most all the registrarion/insurance issues.

I’ve considered that too- particularly for riding Asia. I do wonder the logistics too of a foreigner buying a vehicle and getting all the paperwork in line, and selling it then; but it might still be easier and cheaper on the balance

Have You checked the validity of registration/insurance? I'm kinda curious about doing it the other way and trying some BDRs) maybe in a few years)

I regularly see groups of riders with European plates on their bikes, traveling up and down Highway 1 - a very scenic and twisty road up the California coast. So, it must be doable without too much problem.

Generally you pack it into a crate and fly it. You may well need a Carnet and/or other paperwork, and it only makes sense for longer trips.

Consider buying a bike in Europe instead - generally cheaper and you can get something more appropriate to the terrain.