Though at least a couple of authors managed to be sneaky.
Yep!
I think the first one I caught was when Kyle popped up in Marvel's run of post-TMP comics. That licensing contract supposedly restricted them to characters who'd been in TMP rather than TOS.
After Richard Arnold's steadfast efforts in stomping out several authors' ideas for callbacks, homages and Tuckerisms in the early 90s, it made a few of them determined to slip character cameos past RA. Peter David was particularly adept at it.
And very cool re Ensign Michael Howard!