Cody Winchester shows a trick to get curves working as custom bone shapes in Blender.

A neat rigging thing in Blender is that you can designate objects as custom bone shapes. The bone will take on the appearance of the object, making it easy to customize character rigs. That said, it only works with mesh objects.

Use Curves as Shapes for Bones.

With Blender, there is a little trick that you can utilize, according to Cody Winchester, to get curves as bone shapes. The method is to add a modifier to the curve object, and then the bone will appear as the curve shape. You can change the curve in edit mode and the custom bone shape updates in real-time. This works in all 2.80+ versions of Blender.