Hi Dave, no, I went the other way with my CB750, from the flush type to a flip-up Monza cap. I would think going flush will involve cutting and welding the tank, not something to be taken lightly.
The only other way would be to get hold of a flush fitting with the retaining screws, then you could cut out the old stand-up filler neck and drop the flush fit in it's place, but unless you can find a way to recess the screw area it's still going to be proud of the tank surface.