Kodewerx

To partially fill some requests for shininess, nature, ability, etc. you'd need to have some way of generating a personality (it's a 32-bit value, you could probably fish it out from those "change your Pokémon gender" codes) so that it fits a series of simultaneous congruences. For shininess, you will need the high-order 13 bits of your 32-bit trainer ID (which can be obtained by any number of existing codes which give you the high-order 16 bits in place of the low-order 16 bits, or just try and find a shiny Spinda in the wild and have a good eye for spots). These simultaneous congruences are available at any serious Pokémon info website (such as Bulbapedia).

In theory, this should cover gender, nature, shininess, Unown letter, etc. (Spinda spots are uniquely determined by personality, so if the Spinda with a specified personality isn't shiny for your 32-bit trainer ID, you'd have to "trade for it").

For ability, if the Pokémon is generated in DP (wild, egg, in-game trade, etc), the ability is generated from your personality. Then it is cached somewhere else. (This is likely so that Pokémon imported from Pal Park don't suddenly change ability). The ability is re-generated should the Pokémon evolve (explaining why your imported Pickup Zigzagoon can change into a Gluttony Linoone). Because ability is cached (and the game tends to read off the cache instead of calculating the ability), you could technically hack Spiritomb/Sableye with Wonder Guard... (I keep either Scrappy Miltank/Kangaskhan with Fighting attack, an indirect attacker, a Fire Fang user, or Mold Breaker Pinsir/Rampardos just in case).

Also note that the game will never generate a personality in which the higher-order 16 bits or the lower-order 16 bits are all zeroes.

If you want your egg to hatch shiny, or with some set personality-based value, you can always modify the egg after having obtained it (but before it hatches, of course), as the Pokémon is "generated" once the egg is obtained.