Well, I was wrong. Getting the National Dex does not let you move Pokémon between Diamond and a GBA games--only from GBA games to Diamond. I found this out after my search for a way to just get the National Dex, with no intentions of using any cheating involved to assist me in Diamond, in which I don't really have much interest.
HOWEVER...
You're wondering what I found... It is a Diamond/Pearl game save editor. I can change pretty much anything, it seems. Requests from anyone who wants an impossible-to-get legendary thing, it seems I can fulfill.
BUT...
I am a totally nice guy (BWAHAHAHAHAHAA!!!), but my Internet is sort of down right now: The WildBlue people cut our speed by a lot because the total number of megabytes worth of stuff downloaded over the last thirty rolling days is 14020, somehow. I swear it's h4xxing, or a mistake. Even I don't pirate that much... The number is steadily falling, but it won't return to normal until the number is lower than 13600. In a couple of weeks, though, I'd be happy to grant people those darned impossibles, if there are any... I don't really believe in cheating, however, so I'll probably put them at really low levels. Nothing your Lucky Eggs can't fix, though, eh?
ALSO...
I haven't edited anything I have but my disappointing Pokédex so far and still don't really intend to use this tool to my advantage (though if I replaced my Pokémon with oldbies I could raise, it might be more fun).
But man do I wish I could have transferred some stuff to Ruby. I JUST WANT TO START A NEW FILE WITH AN EGG, IS THAT ASKING A LOT?!?!?!!?!?!?
EDIT...
Man, I'm scrolling through my artificially-complete Pokedex looking at them all in order. The oldest ones' sprites made me smile. The second-gen ones did too. The third-gen ones, not so bad...
And to me it looks like those people were SCRAPING for inspiration for these things. All the new evolutions for the oldest ones that had no evolutions at all... it looks like they fed them gallons and gallons of radioactive creamed corn and watched to see what happened, to be honest.