Let me caveat... it has been a while since I read the book, so I could be fuzzy.
I don't remember us being told specifically that the banner was something that is necessarily related to the Winter Knight or Mab. I remember Mab saying that no Winter Knight has been able to do this since that one famous guy. She didn't say that the other guy did it because he was Winter Knight, just that he was the last guy who did it that was Winter Knight. One could take that to mean that it is some latent Winter Knight thing that only a super small percentage of them can do. But in the light that Marcone also did this thing, it is easy to also interpret this as Mab just saying that this power to grab this mystical Leadership banner is rare, and the last Winter Knight to pull it off was that Tam guy.
So all I'm saying is that, unless I am forgetting some detail, we don't have a really detailed explanation of exactly what the "Banner" thing entails, or what exactly causes it. For all we know, a person could do this without any formal position (like Baron of Chicago, or Winter Knight), perhaps all it requires is a large scale confluence of Magical Forces and a person who can call and inspire leadership. This would mean it is more likely to happen if you have a title and position, but not necessary.
Mind you, I'm not claiming this is the case, just that we don't have enough info to say exactly either way.