This should be different from the constraints workflow, because the fillets are a derivative of the curves that are filleted, meaning, the input curves drive the fillets. With constraints, fillets may end up modifying the input curves to allow for certain, otherwise impossible radii.
I could even imagine a Grasshopper component that allows you to do on Canvas fillets.
@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.
Iām 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:
