Dave, some countries will ask you to buy insurance and will sell it at the border. I do not have an updated list but it is becoming common practice.
It is fairly cheap, 20 bucks or so for a couple of months.
In Argentina you can buy insurance for around $15 a month and if you ask for a Mercosur extension that covers you in all Mercosur countries, which are Argentina, Brasil, Paraguay and Uruguay. It may be recognized in Chile, Bolivia, Colombia and Ecuador.
The company to source this insurance from is ATM but you can do it at any motorcycle shop quick and effortlessly. Most of these shops would not know that ATM insures foreign travelers, so insist and ask them to call ATM to confirm. Don't take a NO for an answer. ATM Seguros
As you may know, the supervision and enforcement of regulations in most Latin American countries is lax, but if you don't have insurance and run over a pedestrian or injure someone you will be in deep brown stuff and wishing you had insurance in the first place.
One more thing: your "personal liability umbrella" whatever that means will certainly not cover you unless it specifies what you are being covered for. So, if it does not say something like third party vehicle insurance in such and such and such country - or something along these lines - you are not covered at all.
Hope this helps.
- Amazing how many people who call themselves professionals are not professional at all