I have a similar situation but have two concrete steps and adding the post on the step is probably not an option, could break the concrete drilling close to the edge, But the tip using the post mount on the flat is a good tip, I can then add a 2X6 on the rake and mount my handrail to that like you did. I had it in my head that I needed to mount a post to the steps but seeing your idea will make for a better outcome, thanks