I use a Corsair Scimitar RGB Pro mouse and it has 12 extra buttons on the side. In the Corsair Software I've mapped these buttons to Function 13 through 24. Now, no actual keyboard has physical Function keys above 12 but they do exist and can be mapped.
All of these keys are able to be bound in ArmA III and work just fine. The issue is that they all display as "UNEXPECTED_KEY_ID" in the controls menu making it impossible to tell what key is what.
The text just needs update to read as F13 through F24 for their respective keys. See the screenshot below for an example. All three of the actions are bound to different keys but they all display the same.