Why set it during TOS when you can set it during TAS? I want more TAS-era novels!
Arex meets Shaun Christopher. Then M'Ress shows up. It writes itself!
Well, according to David Gerrold, Arex and M'Ress were already aboard before "The Trouble with Tribbles". When he wrote "The Galactic Whirlpool", he gave them a cameo (and a feature role to Kevin Riley) and, at the end of the story, Kirk decides the whole crew needs a well-earned rest and the ship makes its way to a quiet shore leave... on Space Station K-7.
Now, although many timeliners put that novel into a TAS-era slot, I once asked David what was his intention for that ending, and he said it meant that Arex and M'ress were always on the ship.
I like that idea. I've always liked to think loads more cool aliens were lurking unseen below decks or just off-camera in Trek, only being seen when budget allowed, like in TMP and STXI.