Animating the sortingOrder property of a Renderer

I'm writing an importer for Spriter projects and I'd like to be able to animate the sortingOrder of a sprite using Unity's animation system.

Curiously enough you can't add keyframes for it, neither in the editor, nor via code. Does anyone have a workaround for that? I want to avoid writing a script that does it, because everything else is working fine via the native animation system so far.