That would be cool for spectator mode but I don't think MCPE has that capability to fly in survival without any mods
changing the gamemode directly without kicking yourself out makes all players online freeze and requires an app restart.
A Bukkit server is a PC server. I got myself right. Because PC is written in Java PocketMine isn't. PocketMine + BigBrother is. Spoiler: the least funny joke ever PC server - BigBrother is PocketMine. PC server - PocketMine is BigBrother. Just a bug, not long term. Not sure whose fault it is. Try out a magic carpet plugin. I'm (or We, @LegendsOfMCPE are ) writing one called Vehicles. /gamemode <player> 3