Well, you don't have to huddle, but that doesn't mean that you can just run a play whenever. On all plays, an official must make the ball ready. There are also rules about giving the defense sufficient time to make player substitutions, but they don't apply if the scoring team does no substitutions.
The NFL rulebook does not give procedures for how the ball is handled after a touchdown. But you have a few things that just aren't a problem during a normal play:
- All scoring plays are reviewed by a booth official. They're not going to start the try until that review is complete.
- The scoring team has to indicate where the ball is to be placed.
If the review is completed quickly and the official makes the ball ready, then the team with the try is allowed to start the play. I'm unable to find anything that creates a minimum time before the ball must be ready, but it does seem it would be difficult to get a quick snap off after a touchdown.