At first when I saw the title of your post I thought of Romulus the legendary founder of Rome, so I thought, "A pagan Saint?"
Which might not be out of the realm of possibility given that there were many Jewish OT saints who lived before the time of Christ, so there's no reason why there couldn't have been pagan ones as well. Given that when St. Paul visited the Areopagus he seemed to imply that there were some pagans who already worshiped the one true God but didn't know it yet. But officially recognizing any particular pagan as a saint would be going too far I think; isn't the usual belief that their names are known only to God?
Never heard of Romulus Augustulus though. Interesting, although there are some who speculate that he was an usurper and the lawful last Western Roman Emperor was a different man.
Also, is there an official Orthodox teaching on Charlemagne? I've seen a lot of criticisms of him from Orthodox authors, but he did live prior to the schism. Although neither his empire nor the Byzantines were ever interested in uniting with each other, so it doesn't seem likely there could have been a unified Roman Empire emerging in that way.