Oh, for pity's sake. I posted a clip of the actual scene above. Watch it. McCoy does not gesture toward his head at any point in it.
I do seem to recall in one part of that scene he sort of moves his head, I believe when he's asking for permission to come aboard or something after he's already complained to Kirk. But I honestly never read anything into that. To me it seemed he was moving his head in the direction of the intercom or something along that line.
But I interpreted the scene literally. He even said, in simpler language they drafted him. I wouldn't read anything more into it than that. Why would Starfleet resort to something else?
In Spock's Case I believe it's clear what's calling him is the consciousness from space, V'Ger. Not Starfleet. And Spock wasn't 'drafted' like McCoy. And in that early scene on Vulcan that is before Sonak is killed, so Starfleet wasn't even aware yet they would need a Vulcan Science Officer (as Kirk preferred)