F20-F24 keys do not respond, but are mapped OS X Yosemite, 10.10.3, MacBook Pro mid-2012.

I am building a keyboard, and have mapped onto it all F1-F24 keys using tmk's firmware. F1-F19 all respond (I'm using Karabiner's EventViewer utility to detect keypresses), but F20-F24 do not. I know the keys work physically, but OS X doesn't recognize them.

Is it even possible to use F20-F24 on OS X, or are there other keys unused by OS X that will work (I want to use them as macro keys)?
Here is the guessing answer, but makes sense, so probably possibly true.
What&#39;s peculiar about F21-F24 keys on MacOS?