Nice post and good thinking, it's always good to see someone thinking of a new and simpler way to try and achieve something. I've read a LOT of posts on this issue over the years (E-Body tire clearance) and don't ever recall someone pivoting the leaf spring in the way you mention.

IIRC, most people have found they can accommodate 275 or some even 295 without alteration, but it is certainly true that it depends on a number of factors that might differ from car to car.
I'll put this on my list of options to consider when I actually come to spec and buy my wheels/tires - top of the list so far had been a Dr Diff relocation kit (rewelding perches) with stock axle.

Nice one and thanks for posting