First of all this is a general business question and shouldn't have been posted here in the software section.
This section is for discussing software.

But, I'd try contacting the Timber Framers Guild business council and see if they can help you find an insurance company that understands timber framing or timber framers.
I would hope there are these type of insurance companies out there.

Good luck, Jim.

PS. I don't need to be told that I'm not the moderator here, but the moderate has a lot to do and doesn't always see all the messages everyday.