Hi Andrew,
Yes, as you have already discovered it is possible to draw a vector in a different program and use that as a widget.
However, doing it that way creates a lot more steps and is limiting. ( you would be limited to a 2d shape)
Plus there is no easy way for you to edit your existing shape in back in illustrator. You would still need to edit it in Blender.
Perhaps you can draw your shapes, then import them into your own library with the Bone Widget Add-on. Then you can add your specific shapes with a click of a button. It is designed for meshes though, so any curve will need to be converted first.
