First, you must enter a code at the "Press Start" screen. This will give the Mega Buster a weapon meter.
Code: At the "Press Start" screen, hold Shoot and Jump for about 6 seconds. There will be a chime to let you know that this worked.
Hadouken: Once you have entered the code, press (down, forward+Shoot). You can charge the Hadouken for a Red Hadouken.
Genei-Jin: Once you have entered the code, press Shoot+Swap1. This activates Genei-Jin, which uses the Mega Buster's weapon meter. I am not sure you can deactivate the Genei-Jin; just run out of meter or die.
So, this is what I know. I really wonder if there are other hidden moves in this mode.
I thought this was going to be a troll. But no, it worked! XD Actually, I'm pretty sure that this is not Genei-Jin, but Sei'ei Enbu, Yang's super. In fact, Rose does this same thing in the Alpha games. Geez, this game has so many neat little cheats. How did you find this out?
http://myanimelist.net/profile/FFandMMfan I'm gonna tear it apart and rebuild it twice as good! I-I'm gonna RENOVATE your ass! - Kanji Tatsumi
It is indeed Sei'ei Enbu, but I figured most people wouldn't recognize the name of that move. People recognize Genei-Jin though (and it looks like it even if it doesn't work the same way).
I figured it out through dumb luck. I was starting up the game when my wife called, so I alt-tabbed the game (apparently while I was hitting Shoot+Jump) at the press start screen. I heard the chime even though the game wasn't maxed. So, this told me that it had to do with the Press Start screen. I figured it had to do with either waiting or holding a button, which led me to try holding every combination of button I could think of. Then I just had to figure out how to do Genei-Jin (hadouken was obvious). So, I pressed buttons randomly. YAY!