I have both run and participated in Pre-Gen games IRL and really enjoyed the heck out of them, even with systems I was already familiar with. It gave me a chance to be someone I wouldn't normally think to play. The problem with this in regards to PBP games is that a person is going to stuck with a character possibly for years if the game is successful, rather than just four hours or so.
Here is what I would suggest for making this game a PBP success:
1) Get players that you know and trust to be consistent posters and good at developing characters. Invite people to the game specifically rather than opening it up to all and sundry. Although a lot of folks might see the appeal during the game ad process, if some of them lose interest later you may be out a plot-central character.
2) Work with your players to create their characters for the role you need them to play. Don't give them free reign with the fluff, but don't box them in either. Folks invest a lot of time in character creation and that helps them feel invested and interested in the game, so I don't think you should skip that. One problem I've seen with pregens is that there will be skills spent or feats taken that I think aren't right for the character or won't be useful. By building the characters with your chosen players, you can explain why they have certain abilities and take the player's opinions into consideration.
3) Talk about, but do not force, the character's progression. Once you hand it over to the player, the character should be theirs, and if they decide to cross class their warrior into a few levels of wizard (or whatever the equivalent is in the system you choose) then they should feel like they have the freedom to do so, even if you thought the next logical step was knight of the realm. Part of the fun of roleplaying is letting your characters grow and change. Pregen characters shouldn't lose that.
That's my two copper pieces.