How do you coil tap a P90 :?
Anyway, if you wanted to retrofit a melody maker with a neck pickup and a scratchplate, you're looking at £80 for the pickup and £25 for a scratchplate that size and shape, so you've spent £105 before you've even paid a luthier to rout the new cavity and fit it for you, as well as fitting the switch and 2 new pots and knobs (another £20), so you're looking at the best part of £200! How again is it not worth that?
I really hate this weird "one pickup is more rock" reverse snobbery. What are you really saying? "I'm going to intentionally limit myself as a musician because it's more rock" ?????
Bottom line is that you cannot get the same sounds or qualities from a bridge pickup as you can from a neck pickup, and some fantastic rock sounds have been created using the neck pickup, just ask Clapton, Hendrix, Page, Blackmore, Slash etc.