Hi Phil
we drove in south america 2 years a truck from cali with expired plates and have had never a problem even to insure it. This was 1 1/2 year ago.
Only Honduras tried to make up a problem. The registration was 2 weeks valid to this time and we told them we would not come back. They let us pass.
If it's possible to do it out site the US I don't know. For cars from California I guess not. They need a smoke test to get the registration.
Hope I could help you.
Have a nice trip.
Cheers

" is there any way for us to extend that registration from outside the US?"

The current owner should be able to answer that question - or is he not a resident of Nebraska either.

--
Tony Lee
Photos at https://picasaweb.google.com/114611728110254134379
Travels map at https://spotwalla.com/tripViewer.php?id=5cfc50ef7ac22ca2d&hoursPast=2400&...

Ooops - should I have answered in English? I'm so used to this being a German Forum. Anyhow - no problem, we drove 3 years with TITLE document only, and sold vehicle to a Chileno, who got it legally transferred into Chile. Take tags (the month/year) stickers off your plates. Only problem might be if you want to drive it back to the USA, first country to check might be Mexico...

Wir sind fast 3 Jahre mit abgelaufener Oklahoma-Registration in Lateinamerica gefahren = kein Problem. Selbst in Chile an einen Einheimischen verkauft. Wichtig ist, dass Du den original Titel (~FZ-Brief) hast, mach 'ne Farbkopie, die reicht normalerweise an Grenzen, und verstau das Original sicher! Und solltest Du Monatsaufkeber [May 2014 oder so] auf dem Nummernschild haben, dann mach die besser ab... Muss ja nicht jeder sofort sehen!