Okay, so I bought an Action Replay, mostly for this, and I have to say, it's a fun little easter egg, but not worth buying an Action Replay if you aren't going to use it for anything else. Fortunately, you can also cheat in other games, so if you've been thinking about getting it, it might be worthwhile.
To confirm what the AR code does:
-EBA replace the Ouendan in all levels, both the Black Ouendan and the Blue Ouendan
-Divas replace the Cheer Girls in the same way
-In the final two stages, EBA replace the black team, and the Cat leads the blue team
-(I haven't had a chance to check the multiplayer, sorry. Presumably it unlocks just J and the Cat as playable)
What the AR code does not do:
-It does not change any of the 2-D cutscene art
-It does not change any of the dance moves. It's cool seeing the EBA do the Ouendan moves, but now their microphones (yes, they're still holding them) are even more useless
-It does not affect the "idle" pose, so the EBA pose differently based on which Ouendan they're replacing
-It does not replace the voices (although I wonder if it might do so in multiplayer, since it's just J, and his voice was apparently on the cart)
-The EBA will not appear on the title screen or difficulty selection screen vignettes.
If anybody has any questions or wants me to try something, let me know. Also, if anyone can confirm that the "official" patch does anything different, I'd be interested to hear what. I'm pretty sure the AR code just triggers the "Unlock EBA Mode" button, so I doubt there'd be any difference.