He's a hitting coach. I think people are over-estimating the role and the scruitiny of the position. The media doesn't have access to the coaching staff after games. Only the manager. They can talk to him before the game if he agrees, but he is under no obligation to do so.
My guess would be when Spring Training begins McGwire will be introduced at a press conference. He'll read some carefully scripted prepared statement which vaguely describes what he did and why he did it. He'll apologize, he'll say I'm not talking about it anymore, and that will be the end of it.
This idea that there will be some media circus surrounding the Cardinals in every NL City is ludicrous. Outside of a handful of fans at BP no one will even get a chance to boo him, as the hitting coach never leaves the dugout.
I'm fairly confident the guy will be a tremendous hitting instructor provided they take care of the elephant in the room the second they get to Jupiter.