Since I've read the Amber series, I now wonder if the card motif on the T-shirt hints that Mac can be associated with playing cards. In Amber series, each of the major characters that have traveled either the Pattern or the Logrus can be depicted on a Trump card. Using this card, you can mentally contact them, reach them so as to summon them to you, or ask them for shelter to bring you to them via the Trump card. Since the shirt depicted in T-shirts are all spades, and it is known that is some card games this is a trump suit, there is a "trump" connection being made between Mac, McAnally's bar, and cards.
So, I'm wondering if Mac is a Trump character from Amber, likely either Corwin, Merlin, or maybe even Corwin's father Oberon, though the last likely not but possible. These Amber characters need not be same as Dresden Files ones.
Now, if we go with Mac as a Amber character either Order or Chaos or both, then he is likely one to be engaged in high-order multiversal events. He could feasibly do things in several times all at once, etc. In some parallel world, Corwin is even considered a Archangel.
So I'm essentially arguing that Mac may be a character from another series altogether. Jim has blended him in to his.