With a registered bike, you must have the required minimum 3rd party insurance. This costs 323 baht for small scooters and 645 baht for bigger bikes. Not exactly sure where the rate changes, but 250cc bikes pay the 645 baht. This insurance covers medical only, for you and the other party in the accident.
Previously in Chiang Mai, all companies were allowed to sell this insurance. Now there is only one company licensed to sell it - RVP (
http://www.rvp.co.th). They have people selling it at the Motor Vehicle office at Nong Hoi, but there you would need to show a green book. They do have a main office, on the south service road of the Superhighway, just west of Tesco Kamtien. They may sell you insuance for an unregistered bike there.
1st class insurance is available, but limited to newer bikes. Usually the dealers can organize that. Rates will be 15-30,000 baht depending on the make and year.
A good compromise is called extended 3rd party. This gives you more medical coverage, minor coverage for property damage (not your bike), and a bail bond allowance of 200,000 baht. I get mine from Viriyah. It was 1600 to start, but dropped by 10% each year of no claim. No more discount once you hit 50%.
As far as it working, I can only talk about a minor accident 8 years ago. My bike had no plate, but the importer was completeing the process. The police wrote it up as "having no plate", but nothing else was done. The insurance company paid everyone's medical bills.
BobS