There are a few solutions to this, some of them better than others. I am working on one which will solve this problem pretty much permanently, without any additional draw calls.
If you're using perspective cameras, one sure fire way of fixing it is to use two separate materials for the head and the body (with two separate renderqueue values). This does work, and with the latest 1.70 beta, you can actually set up multiple materials from within the interface making it really easy to implement.
If you don't really animate these sprites a lot (i.e. using sprite animation, not moving them about), and draw call overhead is a concern, you can simply use a solid shader, and draw the pixel outline in the sprite editor (again in 1.70 beta 2). This means there will be more triangles per sprite, but sorting errors never happen with solid objects. Animating sprites using this technique means allocations when the sprite is switched, but this may not be a problem in your project and may work out better. (Again, another balancing act)
Another approach is to use 2 cameras, one perspective for all your 3d stuff, and another 2D one without clearing z, positioning the objects at the correct z depth and scaling them correctly. This is actually much easier to work out than it sounds, and you get perfect sorting, every time.
Ultimately, this really depends very much on your game and how you need things to interact on there.
If you'd like to discuss your needs in more detail, feel free to email support at unikronsoftware dot com, and I can perhaps offer some more suggestions once I understand your requirements.