Doing some digging and I think it's possible. Just requires your doctor to fill in the Letter of Medical Necessities. The reimbursement part seems like a pain. You have to mail it in each time to after you pay for your expense (please correct me if i'm wrong) and they'll sent you a check reimbursement. Saving 30% on CFS/ME related supplement would definitive give me a reason to use FSA.
Reimbursement form wouldn't if the items are under the FSA eligibility list. Items such as oxymatrine or supplements are not on that list. This would require a LoMN and then reimburse. Prime example is Amazon doesn't take FSA cards but they have FSA approved items. You would purchase with your own credit card and then get to receipt to them for reimbursement.