Houston, I am from Dallas, so good to hear from you.
R-105 does and always will have an inherant problem with paint adhering to it. many old timers state that the wood never did thoroughly dry thus releasing sap and your paint. I once had a 105 siding home and I was always sanding and painting.
James Hardi with Hardi plank is one hell of a smart person. I wish that I had invented this material along with caulk.
Hardi plank has many advantages, hail proof, rot free, real looking wood, and last for years besides will not burn. The disadvantages, are, you cannot use a d-clip headed nail on it. Screw holes must first have pilot holes. This is the only disadvantage that I know of.
Your sanding question, lead poisoning is airborn, so sanding will cause big problems along with the paint chips. Take care.
Now, you and I know the contours and shape of R-105. The only question that I would have is how would you cover and secure the plank to the curved area? If you put pressure on the plank over the 105 curve, the plank will break since there is nothing but air behind it.
If it was my home and I was doing this and also being a contractor, I would screw up ply wood all over the home where the 105 is located. This would give you a secure and flat backing all around. Then I would hang the planking.
I would use 1/2 inch plywood.
Let me know what you decide to do and others suggestions.