A friend of mine just went through this. Although in his case he was ahead of the game
and his 6 month stay had not expired.
He was coming up on 6 months and had no time to move the boat out of Thai waters. The Thai "official" first offered to stamp the paperwork that he "had" left and come back even though he hadn't. My friend said this was totally unacceptable. He worked up the chain of command a little and got an "official" extension (without leaving and no bond) including a groovy three colored stamp ;-)
All above board. In your case I am not sure if you can do this either a) remotely or b) after the fact.
Fundamentally they are interested in you paying import duties if you are staying longer than 6 months. Make sense until you are on an extended layup
. You maybe be able to negotiate a refundable bond which would also make sense. They just wat to assure that you are not going to sell the boat there without having paid duties which is their right as a government
By bonding the boat, they hold the money
and refund it when you leave. I don't view this a registration issue.
Thailand is very cruiser friendly and they hold several regattas during the year. 6 months is a long time to be in their waters, though.