Winston
I ordered some 120 grit "Aggressive buffing compound" from McMaster-Carr. The abrasive size is 120 microns -- much bigger than the 5 micron stuff I was using earlier. It came in a giant 3 lb sausage-shaped tube. I really only wanted a couple ounces of the stuff, but this is what I got.

This time I used it on a Dremel, with a 1-inch diameter felt wheel. It threw off some sparks when it contacted the metal.

Not the prettiest result, but it actually worked. I then worked the back of the blade on a Shapton Pro 12000, and this is what it looked like:

Only the edges contacted the stone, which is exactly what I wanted.

I probably used the Dremel on the blade for around 5 minutes. I first used with a 0.5 inch felt wheel, and it was way, way less effective than the 1 inch wheel. I think it loaded up and got hot a lot faster than the 1" wheel. I'd bet that a larger wheel would work even better.

One more thing I should mention: It's probably not a good idea to do this for bevel-up planes in general, since it'll reduce the contact area with the bed. This blade is actually for a (bevel-up) block plane, but I only used it for testing to see if this would work -- I gave away the plane a while ago.

If anyone else decides to try this, I'd like to hear how it works out!