Probably, all he needs to do is ask one of the major powers in th Balkans, Otto's Hungary, or Venice for support and I can see him overthrowing the emperor in Constantinople. After all that's what did happen in the late byzantine period what with al the civil wars and infighting. Though even if he does lead the empire, Byzantium Is still done for. By 1400 even with the Timurids destroying the Otto's their was no power in eastern Europe or Italy that as capable of halting the Turkish advance especially on a city like Constantinople which was surrounded by the Ottomans.Is it at all possible for Roger de Flor to take the Empire in the 1400s?