The whole thing about certain Pokémon being set as you start your save file is BS.
Also, you can't soft-reset properly on Emerald.
It is not totally BS xD. With every Trainer ID and Secret ID you get your Shiny Spreads.
They determine, on which Seed, on which Frame a Pokemon with determined IVs,
will be Shiny or not. So your Shiny Spreads are determined from the start.
While in Gen 4, you can advance a Seed through specific actions, the only thing that advances your Seed
in Gen 3 (Emerald) is TIME.
It's been a while since I RNGed anything, so I might be fuzzy on the details.
But : Every Save File, has it's unique Shiny Spreads.
You guys should really read about Pokemon RNG. And no it is not cheating, although it manipulates Game Mechanics, but by doing it in a legal and cheatdevice less way.
While this is Gen 4, I RNGed a Shiny Adamant Dratini on my Retail Soul Silver with 31/31/31/30/31 IVs. And many more. Soo, not only it is possible to get without a fail a shiny, it is not that difficult too.
Beldum, is the same case as Shiny Adamant Dratini. A gift Pokemon and Method 1.
The only difference is that you probably will have no methods of advancing your frame. So you have to wait. Other than that I think it was pretty much the same principle. Check out Smogon.com, it is all noted down there.