Thought you were talking about the Buster upgrade for some reason.
Never knew about the Hadouken to be honest.
It isn't normally on the platform as I've gotten to the platform countless number of times. Apparently you'll have to have beaten all 8 Mavericks, have all hearts and E tanks, all upgrades, (EDIT) and touch the platform 4 times in previous play throughs. Since I've always done Armadillo in the middle, and never died 4 times before going into his room, I had no idea it existed.
Just makes me like the game more because all this secret stuff went unknown at that time unless you really knew someone that knew someone that told you their cousin...