My wonderful wife bought me an "Ideal" tire groover for my birthday. It came with a #4 tip and blades which cuts a 7/32" round bottom groove. I put it to good use freshening up my KM2's for winter.
Before:
After:
It took about 1/2hr per tire, but the results are exactly what I was hoping for. I was going to flip the blade and use it to sipe some of the blocks, but I think I might order the #1 tip and blades for that job instead.