So my Wife just called me from the SkyClub, she beeped while walking through the metal detector and the man said you are going to have to go back through and use the Body Scanner.
She said, no worries, but I am unable to raise my left arm, which lead to a pregnant pause (as described by my Wife) and then the TSA Employee calling for a Supervisor.
Apparently the discussion went something like, she is a random for the Body Scanner, but she is unable to raise her left arm due to a medical issue and she has a note from her MD. The Supervisor said but we were told if randomly selected opt-out is not allowed, to which my Wife said I am ok to use the machine I just cannot raise my left arm and in the past I have used the machine with my left arm lowered.
Apparently there was more pausing and then well, have her use the machine and keep her left arm lowered and manually clear the left arm which will light up on the machine and with that the Supervisor walked off.
So, all I can say for sure is that if you are willing to use the machine, but are unable to raise an arm (for example) you might still have to use the machine but will be allowed to keep the injured arm lowered.