Alright, I tried this and it worked. For people who might be interested in getting legit/legal (whatever - you know what I mean) event pokémon from the B2/W2 era, I recapitulate the main points ...
As PokeGen 4.0 is not yet released, mystery gifts using (AR) codes does not work up to now. So you can't use the event database of ProjectPokemon for the real DS games. An alternative step is to get the needed event pokemon from a source somewhere else, for example Pokedit.com where you can download PKM files for each of them. Afterwards you can use PokeGen to create a code that puts the pokémon in a specific box and slot. The problem is that those PKM files are edited in terms of the IVs/EVs. Take a look at the "Plasma Genesect" for example - it will have almost perfect parameters. Is this a problem in terms of beeing legit/legal? Not really, but it's barely authentic in the end - especially when you imagine the case that a lot of people might be doing this and therefore you'll have the exact same pokémon as everyone else.
Thanks to Kaphotics for the help, because there is a workaround where you can get those event pokémon with a little bit more work, but they will be a lot more plausbile eventually. If you play the game on the computer using a ROM, you can trigger the mystery gift with the current PokeGen version. It worked fine for me using the right language version, there is probably no need to use the japanese version. Start the game, play until you're be able to go to the Pokémon Center on your own and save using the normal ingame menu. Load the savegame (depends on what emulator you are using - for DeSmuME they can be found in the battery folder) into PokeGen, load the needed mystery gift file from ProjectPokemon (PGF) as well and save everything. Restart the Emulator and go on playing the saved file and you will be able to pick up the event pokémon in the Pokémon Center. After receiving the event, save your game once again. Now you can load the savegame one last time into PokeGen where you can get the data from the event pokémon and create a (AR) code, so you can "transfer" it to a boxslot of your real DS game.
What is the advantage of this more complicated way? While triggering the actual mystery gift, the stats of the pokémon will be created by the game code itself. So it will be legit/legal AND RANOM in the first place. The randomness is what I was looking for, because now you have not an event just like everyone else but your own one. This fits into a collection of pokémon in a much more plausible way.
Just for completeness: Both approaches are working though, and you get legit/legal pokémon that can be transferred through Pokémon Bank all the way up to generation 6.
Thanks to Kaphotics for the help one more time! I hope I made everything clear if someone will be searching the forum for a similar issue again.
Have a great time & Best regards!