it's quoted on wikipedia that he said sometime in the 60s or 70s that he thought the Enterprise had Marines on board, but it doesn't say he never figured out how to use them. christ, they could've easily been used in Errand of Mercy or even The Trouble With Tribbles. or they honour guard in Journey to Babel could've been given different uniforms as Marines...
There you have it. Considering the problems TOS had budgeting for normal uniforms (and just about everything else), it is doubtful if they could have afforded name-tags, never mind anything more.
So it sounds like in TOS it was probably money that killed the idea more than anything else. But, by the 80's, GR's thinking had taken .... a different path, to put it nicely.