Vanguard from the PMBR said, "BrawlEX is not 'feature complete' yet. Aside from the fact that it wasn't compatible with the heavily modded code and fileset that is Project M out of the gate, it didn't support cloning certain things like soundbanks; essential to crafting the polished experience we want to deliver."
http://www.reddit.com/r/SSBPM/comments/2645f1/how_brawlex_can_have_100_new_characters_yet_the/I assume it has to do with Brawl getting upset if the filesize is changed. I'm sure we'll circumvent this problem sooner or later.