What do you mean by nationalism?
In the Middle Ages, there already was a degree of Greek cultural chauvinism in Byzantium, at least among the ruling classes. I don't know what promoting that further would do, other than alienate the Armenian, Slavic, Vlach, etc parts of the aristocracy even further.
If you mean a popular nationalism, as in a proper ideology -- well, your average Greek shepherd didn't care much about the literary traditions of Homer, the discourses of Socrates, or the imperium of Alexander or Constantine. The closest thing to what we would today call a popular "ideology" would be religion. And they already had a religious identity around which political and military identity could be organised (Orthodox Christianity), and even an institution to organise those political and military identities (the Byzantine Orthodox Church).