Well, the starting pokemons' stats are in their script, so there you can make them shiny. As for other pokemon, wild pokemon obviously are random, but you can use one of those game shark cheats and make them always shiny. If you find out what the game shark cheat does and it is in rom code, then you can edit that section of asm code to act as if the cheat was present.

cYa,

Tauwasser