It is possible for a foreigner to buy a bike in South Africa or Namibia and travel most of Southern and East Africa from there, I know somebody who went as far as Kenya on a SA registered bike with no carnet.
The same in most SE Asian countries although if you particularly want to go to Vietnam you have to buy the bike there as foreign vehicles are not allowed in but you can take that one out to the rest of the subcontinent.
India you cannot technically buy there but there are ways around it involving it being registered to a local or you can rent.
South America you can buy in some countries, I am not sure which ones and then take it to others, no carnet needed there.
Both Canada and some states in the US allow foreign registration which allows you to go to North and Central America.
Australia it can be done but I am not sure if all states allow it, bloody expensive at the moment as well.
The UK and I believe other European countries allow foreign ownership, finding insurance in the UK is not easy but possible, once in Europe you can travel to Central and Eastern Europe from there.
That is just about everywhere covered so yes it is possible.
If it's in the Touratech catalogue then you don't need it
"You've got to think for yourselves! You're all individuals!"-Brian of Nazareth