For Essos to be orientalist, it would have to be basically analogous to traditional representations of actual countries in what was once called the Orient. It's not. Both the Valyrian subcontinent and Slaver's Bay, for example, owe much more to Rome than to any "Eastern" nations. The problem is that Martin's geography, ethnicities, and (sometimes) architecture create an East-West dichotomy like the one in the real world, so readers treat Meereen as an eastern society even though it's equally Roman. right down to the togas, er, tokars. I think he does that in the hope of undermining the perceived differences-- Dance explicitly suggests similarities between Meereenese slavery are Westeros' feudal culture-- but the disinclination to write from and fully investigate the perspective of those characters make it feel more like an exercise in othering. Which is a better buzzword for this case than Orientalism, I suppose.
I may be splitting hairs here, because I'd be fine with working with the term "othering" as opposed to Orientalism, but I honestly think ASoIaF qualifies, in these ways:
Most obviously is the explicit West/ East dichotomy Martin sets up. Because the primary point of view of the books is Westeros, Westerosi conceptions of Essos are the starting point of the reader. The Westerosi consider the Essosi decadent barbarians who lack the more advanced Western concepts of honor but are nonetheless exotic, mysterious, magical, and enticing. They have bed slaves and sorcerers - two of the key images of Western Orientalist configurations.
Secondly, despite the Roman references of tokars and gladitorial fighting pits, the Essosi are presented as, in the words of Edward Said in his definitive work
Orientialism, complex, but ignorant. In general the Essosi are presented as working within a set of cultural preconceptions which they do not question ("It is known"), while Westerosi characters (at least some of them, like Dany, Tyrion, Jon) frequently question their cultural preconceptions and in their questioning take action which eventually proves them right. They are critical thinkers, but I can't really think of an Essosi character who demonstrates the same sort of thought (I've only just finished reading the books though, so if someone wiht more knowledge of the world and characters would like to correct me, I'd be interested to know.)
You could argue that Martin's use of an ancient society like the Roman Empire as his working material for the Eastern metropolis of Meereen, as opposed to the 15th century references of the War of the Roses for Westeros implies a bit of historical superiority - in general Western readers at least might perceive a culture 500 years removed from us as more advanced than a culture 2000 years removed from us, even if both are direct antecedents of our present day culture. And it's not as if he's chosing for his Roman references the things we revere about ancient Rome, but rather the things we ourselves reference as silly (togas - perhaps the most impractical garment ever conceived, which is exactly the point made over and over again about tokars) or barbaric (the arena).
Add on top of all that the Dothraki who glorfiy murder, mayhem, and rape and are the readers' introduction to the cultures of Essos, as well as being obviously based on the Mongols, the hordes of which have merged in many a Western mind with modern Islamist fundamentalist "hordes" screaming about the great Western Satan, and I think there's a case to be made for using the term Orientalism.
All that said, Martin manages, intentionally or not, something of a critique of Western imperialism through Dany's misadventures in Essos, beginning with Mirri Maz Duur's revenge upon her. Here comes the young queen, following her soft heart and magnanimously choosing random women to save from rape, to bring them into her household as servants, when by her marriage to and love for Khal Drogo she is ruler of a culture built on violence, slavery, and rape - a rather classic construction of the paternalistic conquerer from the West. Mirri Maz Duur points out her sheer ignorance of reality by sacrificing Dany's child and giving her zombie Drogo in return - "Why don't you take a look at your khal? Then you will see exactly what life is worth when all the rest is gone." Dany's experiences in Astapor, Yunkai and Meereen are in many ways larger replays of this same lesson, and there may even be a fairly good parallel of her "I'll bring freedom at the business end of a dragon and all will be well" but which rapidly devolves into utter chaos which she cannot control to say, oh, a Western power deciding to march on a Middle Eastern country to "bring freedom" and ending up mired in a chaotic 10 year war.
