I've found a workaround which may be helpful for debugging. The yellow issue disappears when you add a layer with either "Hard Light" or "Subtract". There doesn't need to be anything in these layers:
eg

From my understanding auto render doesn't work when you scrub the time line as it's a means of keeping the scrubbing process or animation playback smooth for the end user.

Ah ok, for some reason I couldn't find in the search...

Yeah especially since we've been waiting for it for years, its a shame to see it archived like this... Unless nodes come in the next couple of months

I meant to put in c77eefb for the Blender version I'm using.

Ok I think I found the source of the error. In the bones "HeadTurnTowards", "HeadDefault", "HeadTurnAway" There is a constraint that limits the scale of a maximum of 0,0,0. This is so I can switch between bones, (in)effectively hiding it to switch between the heads. But as I feared having a scale of zero could have some possible mathematical problems. In this case it came out as an error in the FBX export.

Turns out if you go to the HeadControl and delete the custom property "HeadSwitcher" it exports without errors

Great, looking forward to it

Hello, just curious on any updates? If there's anything I can help with or help test, I'd be glad to help out.

I will watch this closely, I look forward to it getting added. Have you tried blender's IRC? Seeing as adding this should bring Blender's animation tools closer to the level of Maya's, I think it should get the attention of some developers.

If it's ready, I hope it will be added to trunk soon. Many rigs will definitely will benefit from this feature. Maya and other software already has this feature, which allows for some pretty interesting things to be done as you already probably know. But yeah, I hope it will be added to trunk, there's lots of good work there.

I'm currently working on a project, this modifier will greatly help with the animation rig I'm working on. Has there been any additional progress in this area?

Yep! it seems to be working now thanks!

Ah sorry about that, yeah its 2.70. The build I tested was updated about 20 hours ago, and I didn't see any fixes in the commit log that addressed the issue so I posted it here.

Forgot to mention, for the exact steps. You have to open the untitled.blend.

will this be addressed in the future? having transparency is extremely useful when working with camera projection to line the objects up properly. Also with Jonathan's example of creating characters to align with the image reference.

Ah dammit I feel silly, must be because its 4am. Thanks for the help.

Oh I forgot to mention my specs: