Hi-
Hate to horn in, but I was a state officer in Montana for 12 years, and the guidance in Montana Code and case law was that even if a vehicle had been originally delivered without seat belts, if it had subsequently been equipped with them, their use was required. Failure to use them opened the operator to issuance of a citation.
Now unlike what you may have heard, issuance of a "ticket" is ALWAYS at the discretion of the officer, so if you are good at throwing yourself on their mercy, you might come away unscathed...might.
If there is a provable physical disability/condition which prevents use of a seatbelt, then medical certification of that condition will generally trump the law and I have met few judges indeed who will presume to override a medical opinion.
Good luck.