I was going to say, I know Rob Roy wasn't the only one who did this (if nothing else, I really thought it was Lovat, would have been totally in character, and it turned out I was misremembering), and I wouldn't have proposed a Jacobite source just on the basis of that one anecdote, but since Frey is already basically Lovat in my mind, I didn't look too far for parallels. GRRM may of course have had Bosworth instead or also Bosworth in mind (I myself don't remember the battle well enough to confirm your memory).