That might be a good approach! I've sort of let this simmer (okay, okay, I've been doing other things and occasionally feeling guilty about my lack of visible progress, but I have been thinking about it) and I'm somewhat torn.
On the one hand, as a generic concept, I love modularity, and I think it might help get the sort of variety of characters you see in SOIF.
On the other hand, having played some for reals AW, I really really appreciate how the playbooks a) give clear but not overly constraining direction to what the players are about and the sort of conflicts they should gravitate towards, and b) they concisely convey the world in intuitive bits and pieces. With a game set in an established setting, this latter consideration might not be as necessary, but would be helpful for people who get pitched a highly political historical-feeling fantasy game, rather than knowing Martin's work already.
The other AW related thought I've been having recently, and I think will *really* apply to SOIF is that the MC's job is soooo much easier when a) the characters have strong agendas and b) the situation generating moves come into play. The very first session I ran, the Hocus's followers provided loads of juicy NPC-PC triangles, potential conflicts and future badness, and so forth. When I ran again and all I had to work with was a successful operator's roll where the player played it safe and only did one gig, it was much harder.
All of this is a way of saying I'm very fond of building in MC fodder into the character types, and a purely 'pick your moves' modular approach might have the players avoiding such moves unless the potential gain is just too darn juicy to pass up.
So, to come back around to your suggestion, having written this post, I'm liking it more and more. I might make a number of playbooks that then have subtypes (come to think of it, like the monsterhearts playbooks). So you can be a high lord or a bannerman if you pick "Lord", you can be a sellsword or a sworn sword if you pick "Stabby Guy", et cetera.
And then maybe have some optional "make your own" rules akin to the custom moves and what not in "Advanced Fuckery", like the way Lady Blackbird gives you pregens and a super tight opening situation, but later on gives you complete character creation rules.