Holy hell. Yeah, Gen 5 would be literal murder to try and do. I have tried to hack white once and the moving sprites are not happening. Like back in 2012 no one knew how to do it. I have no idea if anyone ever figured out how to add animations since it's entirely in the base coding, or if there's an easier way to import stuff now that it's been around for a few years. Getting Moemon sprites for gen 4-5 would also be frustrating but not impossible. But All said and done, even if you did manage it there's no way it'd work as a patch for pokemmo.
RIP Moemon there's no way in hell your coming back from this.
Edit: looked into it and there are not any new ways to hack the game and in fact even the old hacking tools are gone.