I can't tell you why this thought first popped into my head because it's been so long that I can't even hope to remember the reason. Nonetheless, it was something that spawned from my noggin like Athena from the head of Zeus and for lack of anything better, I thought I'd share it here.
So yes, reading A Song of Ice and Fire as if it were historical fiction is an odd notion and probably not one that occurred to George R.R. Martin when he conceived the series, but it's something that I don't think is that far-fetched. Historical fiction at it's biggest and grandest is simply the retelling (with liberties) of historical events as told from a mix of historical figures and wholly fictional ones. The stage is which ASoIaF is set is not dissimilar to that. If you were to excise the more fantastical elements like the dragons, white walkers, and magic, then you have a retelling of events that wouldn't be that far removed from what you'd read in a historical fiction novel or novels.
Indeed, you could take it one step further and argue that if the plot of the series were distilled down to just the events that occur, then the entire series could be read as a historical record. Political upheavals, assassinations, and wars of succession are exactly the kind of thing that happened in Europe for centuries, just sans dragons, zombies, and Kitt Harrington's perfect hair. In fact, when I finally get around to re-reading A Game of Thrones and (hopefully) the rest of the series, I'm going to try and do so from the point of view that I'm reading historical fiction.