Bones in Source are often right handed on one half of the player model and left handed on the other. Tools like 3DS Max are able to support this by setting the bones' lengths negative on one half the model.
Could create two rigs on import, one for control and one for export, but that sounds really unwieldy.
Could use custom bone shapes to display a custom backwards octahedron.
Could integrate with a plugin that already does this sort of thing like https://github.com/waylow/boneWidget.