I've always had an issue with convention games and fate - the aspects are so personal and important to the character that I find it hard to pick up a pregen and do much with it - I actually had an easier time working with consequences taken in play than anything on the sheet.
This is probably a personal flaw...other folks may be fine with pregens in DFRPG, but the way I've handled this problem as a GM at a convention in games with similar issues is to find a scenario where people could bring in their favorite characters, perhaps reskinned to a certain power level (eg, Chest Deep in DFRPG, 60 years old in Ars Magica, 8th level in Pathfinder or 3.5 D&D) and have it be ok.
You have to have a scenario that provides a strong hook to involve diverse characters, and that doesn't depend on any particular character type or build to succeed (this is actually easier in Fate type games than most). As a GM you also need to have a high degree of confidence in your ability to manage table variation - you may not agree with the cost of a catch or something but I've found it's best to just trust players - if you extend trust most won't abuse it and those that will are going to likely be disruptive in some other way anyway.
The largest issue with running any Fate game this way is that players need to be better at drawing their aspects to your attention. OTOH, they will...these are their favorite characters, sometimes with years of history. My biggest issue GMing fate is keeping track of all the aspects, and not having to remember the players very well is actually a benefit.
DFRPG is a shared world, so it's not hard to imagine a team-up between characters from different "cities" (campaigns), and the never-never allows for inclusion of even alternate reality versions if you need to. Fate Core or FAE, being genre-neutral would need some mechanism like "the locals tried to summon the greatest avatars of law from space and time...and unfortunately for them, they ended up with you guys" to allow some concepts to be at the same table. (in that game, which just used a homebrew system to approximate the characters on the fly we had a Judge from the Judge Dredd universe, a Lensman, a Pendragon knight (they had power of high and low justice), a pulp hero lawyer similar to the Doc Savage aid Ham, a Legion of Superheroes character...you get the idea...players built their characters in play based on what in Fate would be their high concept, but pulled them from strong external sources so most of the table could accept what they could or could not do)
