Re: Bridge P90 and a FR Trem?
It might be a bit narrow, depends a bit on the P-90. I don't know if any pickup manufacturers make a fender/floyd spaced P-90 or how good it is.
Some people have zero problem with the strings not lining up, just raise the polepiece under any string until volume is even. Others find that does bad things to tone balance (bright/thin E strings) and can't stand it.
It's no worse than a traditional spaced humbucker under a wider spaced bridge, and if you are playing with a fair amount of compression, the failure to line up is just cosmetic.
As far as never been done, I've seen it in strats and teles (just not with a Floyd) and they just didn't worry about the strings not being perfectly over the poles.