I seem to recall that Chris Thorman did this years ago for Huddersfield and it wasn't allowed, and that the rule was changed shortly after to allow it. Don't hold me to that though!
Marc Sneyd also kicked a conversion with a drop kick last year, which was allowed. Although the laws of the game on the RFL website doesn't say this is permitted.
Mind you, the laws also state you can drop the ball (or place it) at the play the ball and it's not a knock on, and that certain types of obstructions should result in a scrum!
