The intro level to Super Mario World is simply a flat land with a few bushes with berries on them. After a couple seconds, the intro message will appear. Immediately after this message, Mario will appear on Yoshi's Island, on top of Yoshi's House.
If hacking this level, be sure to not let Mario die or the life counter will warp to "P5" (really 255) after getting a game over. Ending the intro with the glitched counter will send the player back to the title screen.
You can use this level as a normal level in your hack. Just link to it with an exit-enabled object. However, don't forget to push Shift+F8 in Lunar Magic, or the Display Level Message 1 sprite will glitch everything up. You could also make a one-level hack in which this is the game's ONLY level, by deleting the display sprite and never putting it in anywhere else. However, dying in such a hack (or dying in this level at all) will glitch up in the life counter if done too many times.
Unlike with the title screen, you can use exit-enabled objects in this level and have the intro span multiple levels. Just don't forget to put in the display level message sprite at the end!