It only took 23 years to discover! Note that this code only works for the JAPANESE version of the game for the Super Famicom (超ゴジラ). To activate, wait until the opening Toho logo pops up, and press the following buttons in quick succession:
Left, Right, Y, A, Up, X, Down, B
If entered correctly, you'll hear Godzilla roar, and upon beginning a new game, you'll be taken to the Stage Select instead of the first level! As Stage 6 (Bagan) is reliant on the choices you make in Stage 5, there is no option to choose that. Interestingly, data exists in the US version of the game for a Stage Select, but because the opening logo was replaced with a generic blue copyright screen, the code was nullified. Credit to DylanRocket for discovering the button combo, original discovery of the Stage Select data found by yours truly! DylanRocket : https://www.youtube.com/channel/UCg42bY9PP5IBVGKwA1QT1Gg
No copyright infringement intended. GODZILLA and related characters property of TOHO. Footage and music are used solely for the entertainment of others, and to bring awareness to the franchise.