IMHO creating a kingdom in the empire (which stays a part of the empire) is more likely than creating a kingdom on lands, which are considered a part of France. (The elector of Brandenburg only could become king in Prussia (parts of Prussia were still Polish), because one of his predecessor managed to make the duchy of Prussia independent from Poland and a sovereign duchy.)
Furthermore in that period, just styling yourself wasn't enough Papal, imperial and peer recognition would be required
All he'd need to do is definitively sever ties of allegiance with France. Once Flanders is fully independent, all you need is some cajoling to get the title granted to you by either the Pope or HRE. Maybe once Reformation rolled around Papacy would be glad to appease the Burgundians.