My dear departed dad always said never take no for an answer, and that there is no such word as can't.
After 3 days without seeing any where to try and buy insurance, we stopped at Piura before heading across the desert.
And found lots of SOAT offices.
We entered one, La Positiva, and started to ask about insurance, waited to be offered a year and then asked for 1 month because we only had temporary import docs for the bikes.
We showed our temporary import doc and she got on the phone to ask some questions. And said that "yes, you do it, but not here, in our other office a few streets away". he then found someone to guide us there.
All was done in about 10 minutes, both bikes.
The price ? S97. Yes, about 35USD.
Details of where to go
La Positiva
544 Lima
Piura
It is just North of the Plaza Des Armes (where there is also a helpful tourist info office)
La Positiva Seguros
As Piura is probably the 1st major town people come across if heading south from the mountains, this could be a useful stop.
FWI, the coast route does not seem to have any police hassles/stops.