Ok, so you weren't clear about DIY in your first statement.
But how do you define "regularly servicing guns"?
When does adjustment (or routine maintenance) become servicing? How often is "regularly" defined to be? Should the restriction only apply if the person takes payment for the work? How about if payment is just for the price of parts and postage?
Why do you really want to create a new offence for working on an airgun for other persons without RFD status, aren't there enough restrictions already?
Will I find myself saying "Sorry mate, I've changed 3 pellet probe o-rings and cleaned gunk of 2 hammers and adjusted 3 triggers already this year - that's my quota - I'd love to sort out your rifle but any more tinkering and I'm in danger of arrest due to 'RobF's anti-tinkering' law.