wombat2go, thanks for chiming in with input from your experiences. Like I said, I'm going to modify the holder.
Regarding the lens and film flatness bits, this is my point. I don't care about slight variances here and there whether it be film flatness or the particular lens and how it projects across the plane of the film. All I care about is that it makes the pictures that I want. Nothing else matters. The question is simply will a slight modification to the roll film back provide a usable solution? I think the answer is yes and now I've got a nice little experiment to see if I'm right

Ideas from anyone else? There must be someone here who's actually done this, no?