Spanish insurers cover cars in TF.
Is considered a public road... so, said in other words, there is no expecific exclusion to the official road name of the ring (i dont remember which one it is)
(and we now that from pretty first hand... not my car but nearly).
However, your car, should be the last of your worries...
Track time lost because of your incident, armco barriers, spilling oil or coolant and being found gilty...
No insurance against the bill that the private company of the ring will issue against you... and that will likely be the expensive and ruining thing.
Last friend involved (without causing the crash by himself in the first instance) have a pending bill over 200K euros, that is going to be claimed in court...
that happened in a multiple crash, because a posh Mclaren spilled coolant on the floor and about 12 cars were involved. Bill for the Mclaren is greater...
Probably you are already aware of how ruining can be the ring... but just in case.
If you have booked with RSR, ask for Serxio from and say hello from me
My advice for the ring... the cheapest car possible, and if in crash... just abandon it and run.
I'm a big fan of the ring, but has turned out in a crazy and unsafe thing due the hype and trendy moment the track is living...
Stay safe out there.