Well, you can use the Live Selection and use its radius to select points, edges or ppolygon. Just increse the radius value to change the size of the circle selection.
You cannot use a spline as a selection border, but the other selection tools come quite close.
Arndt: I feel embarrased for not “seeing” something so obvious! And I’m grateful for your help and for not stating how obvious it is 🙂
Since most of the time I use the Live Selection tool with a small radius (or with objects with fairly large polygons) I never noticed that tool had a circular pattern! I always thought it just selected an unorganized “bunch” of polygons.
All the best,