That's really cool, but why haven't Sega ever tried putting those zones back into the game for Sonic 2's re-releases (XBLA, VC, Mega Collection, etc.)? It can't take up that much time and effort, surely.
I had no idea about the plaque in Dodongo's Cavern in OoT
However there is a way to unlock Luigi in a half-legitimate way, using a modding tool that allows to dig deep in the game's code.
The Luigi unlocked here should be the one that they originally included for the multiplayer (as mentioned at the end of that GT video). But there are no magic codes or collecting all coins or gooseberry fool like that, it's just about going through the game's original code and finding the right stuff. Anti-climax.