yeah, just got a new lollar firebird pickup in and it has the same thing - i assume the baseplates jason uses are different to the screws he provides so alters them like this. I was surprised he didnt just get some baseplates made to the size he wants but this is still a perfectly fine way to do it
I would guess he makes his Firebird and Minihumbucker pickups on the assumption that they'll be using an LP Deluxe style mounting, so he then has to mod them for ring/scratchplate mounting. I think it would be more sensible to have threaded feet as standard, then let the customer drill the holes out for an LP Deluxe. But who am I to question?
Sounds like it would be easier using a nut than cutting a baseplate. Would I need solder the nut or just let the spring tension hold it together like you say? I've read about the mounting difficulties with springs flying all over the place!
The nut would need to be held in place on the pickup foot, because otherwise it would tend to rotate as you turned the mounting screw (although friction would help).
A couple of drops of superglue might be enough, so long as you were careful not to get it inside the thread.
It might be tough to get #3-48 nuts in a UK DIY store, I think they mostly have metric stuff although I haven't really looked for specific sizes.