Today I fabricated a bracket using 1/8" thick X 1/2" wide aluminum to mount the CB650 fork emblem on my CB550, and attached it using a longer bolt to replace the stock bolt securing the front brake switch block. the width, height, and curvature of the CB650 emblem fits the CB550 forks very well, I dare say it looks about as 'factory' as I could have hoped, I'm very pleased with the look.
The first picture is my home made bracket, curved to match the curvature of the emblem, the outer two holes are tapped with M4x0.70 threads so that the emblem can be secured/removed from the outside without disturbing the bracket, the second and third pictures are the finished installation. If anyone else wants to try this too, let me know, it was fairly easy to do.