> where the extent of the rounded volume is determined by the surface areas in the curve. So the larger the area in the curve, the larger the round volume of that area in the 3D result.

I did not get this part. Are you trying to generate something like this???

1. create a curve, closed or open.
2. create an arc, half circle to be exact.
3. do "sweep" between them; curve as the rail, arc as the profile
4. do "planar" to cap bottom and top openings if your curve is a closed curve.

so you're asking a script which will do things above while determining the highest radius of the arc where sweeped curve will not entwine?