Here's another work-around (no fully tested) that is still ugly. Use two sketches to force the solver order. First create arc in a 2d or 3d sketch on the desired plane. Now midpoint extrude a surface with the arc. Create a new sketch with the style spline and the "On Plane" relation. Add the "Equal Curvature" relation to the arc. This will be over-defined since a tangent relation is automatically applied, and one component of the tangent direction is redundant with the "On Plane" relation. Now delete the auto-generated tangency relation and add it back as tangent to the extruded surface, rather than the sketched arc.
I realize that the math is messy when you have "On Plane" relations in 3D sketches, but I want that relation on the style spline itself (rather than say all the control points) so that I can readily change the degree.