They can appear in any TOS novel right up to ST V and beyond. That very few novelists choose to use them is another matter altogether. Not PAD's fault.
Well, Dayton posted the following about That Which Divides recently, so maybe they'll make an appearance:
Also, and since I'm here, I can drop a small hint about the story. As some folks already know, it's a tale set during the five-year mission as depicted during the original Star Trek series. It's worth noting that this also is the period of the animated Star Trek series. Just sayin'.