Yes, I believe you can deduct the added cost or apply it toward the FSA. This seems to me similar to the situation I am in where I normally use progressive eyeglasses but I have a pair of single vision computer glasses to use at work. Since vision correction is a deductible medical expense, this seems to me to be allowable.
The FSA administrator may have their own rules and requirements for documentation and substantiation of claims. They have an obligation to the IRS to not pay disallowed claims, and if they do pay a disallowed claim, they get penalized rather than the employee who submitted the claim. Hopefully they will agree, or you may have to peel their decision to a senior manager. Good luck.
If you are self-employed, you have the option of deducting work-related accommodations for your medical conditions as a work expense on schedule C rather than as a medical expense on schedule A. Then it would not be subject to the 7.5% threshold.
