Robs proposition does raise some interesting points.
I work on airguns (mainly springers) for club members purely on an amateur basis. Now as anyone who has worked on springers will know theoretically you can easily have in your possession a rifle that is over power because the spring has yet to be shortened to a suitable length. If other circumstances conspired against you (for what ever reason) your premises could be searched and the over power springer taken for testing (you haven't had time to sort it).
Without an RFD your goose is starting to cook.
It's also not unknown for someone to ask you to have a look at a gun for them and you find it's slightly hot. It's you that holds the gun.
No I don't think you should have to have an RFD to work on airguns in an unofficial capacity but you should be careful.