Correct me if I'm wrong here - after you lap a hone, the top surface is flat. On a flat surface, all points lie in the same plane. Therefore, the sides of the hone are at the same height as the middle of the hone.

I don't know what you mean by "bridge," but I have hones that are narrower than a razor and hones that are wider than a razor, and I've never had any issues with a razor only contacting the edges of a hone on any lapped hone. I cannot imagine why a knife with a straight or smiling edge would act any different than a razor with a straigth or smiling edge, but I don't hone knives, so I'll take your word that there is a difference. Knives may indeed need rounded edges, but razors do not. The only reason, based on my experience, to round the edges of a hone is so that you don't accidentally dig the edge of your razor into a sharp edge and damage the edge.