@Gijs, I forgot to mention, could you write up that the radius will have to be recomputed when the input curve is modified? Say I change the angle or modify the CVs of a spline that is filleted with another curve on the intersection. The bulk of the work is usually in re-adding the fillet once you have made changes to the input. If Rhino could do that without getting into full-blown constraint based sketches, that would save a lot of time.
I’m more or less thinking it should work like this proof of concept of a script I am working on in Grasshopper (but with the double click and a widget instead of/ in addition to attributes:
From this topic: Detect geometry changes - access GUID data in between paranthesis? - #8 by Intuos