do the insurance companies have a right to demand I submit a w9 before they consider claims from my client?
No, you have no relationship with the insurance company.
In my view "demanding" your W-9 is a sneaky way for information about you to be passed on to the IRS.
If you don't fill out a W-9
DULY requested by an employer, partner or other entity
DULY entitled to your
taxpayer ID information, you may be penalized $50 for each instance.
You may also be subject to backup withholding, which means the payor will withhold 28 percent of your check and forward the proceeds to the IRS.
NOTE: You APPEAR to have no direct relationship with the insurance company.
I don't see UNDER what lawful authority an insurance company UNKNOWN/UNRELATED to you has the legal authority to
DEMAND YOUR tax information.
You might wish to consult your attorney or CPA to see how best to proceed, as either of those entities would have far more information about you to advise you accordingly.
I'd seek such a consultation prior to doing, releasing, or saying anything about the W-9 "demand".
You can read more about W-9 forms at: Turbotax
What Is IRS Form W-9? - TurboTax Tax Tips & Videos
OR, what the IRS has to say about the matter:
About Form W-9 | Internal Revenue Service
I hope you get this sorted, mate.