Ed, If you go with a thinner Riser then you will have a stronger attachment of the Bullnose to the step. Also consider the Moisture migration from the Concrete. If it is really old structure and well cured it shouldn't be too big of a concern but there will some MC changes as seasons change and this may effect the riser (being a wide panel) Consider Paint or finish on the back side (Not Latex as it Breathes and will allow MC to pass through) Seal the concrete with vapor barrier paint. Or you could use manufactured or stable veneered panel for the riser. Also leave some space under the lip of the Bull Nose to allow for expansion, etc.
You probably know of this but I thought I would just remind you before you set out and then have to say "Why didn't I think of that"
PS.... Want to see pictures of completed job....I really like the Wood Riser and Bullnose with Tiled Tread. Unusual and quite attractive. I think you will find that the PITA of making it all come together is the reason for the "Unusual". I can see where it will be very time consuming but well worth the effort.