Thanks everyone! I don't know if it would be worth the trouble adding the in-game editor from the start (would be quicker to just make a standalone one for me to use), but now I'm really thinking I may add in one for players. It definitely sounds fun! The engine can support it easily, and I wrote it, so either way!
On the subject of the Smash Bros editor, I didn't play much Brawl (played a ton of the original and Melee), but I've been playing the Wii U one. The level editor does not allow for as much "prettifying" as I'd have liked :-(