[bradch00]

Maybe I am looking at this in a too simplistic fashion. It seemed to me that the OP wanted a 16bit file that has been run through a dither plug. So... would it not be simple to (once the mix is set and one wants to create a 16bit dithered file) to create a stereo audio track, insert a dither plug-in, go into Setup > Session.

Set the bit depth to 16 Bit, select interleaved.

Record arm and hit record... the resulting track would then be 16bit, dithered and you wouldn't need to export it...

I have to be missing something here, right?

Of course after you have created the 16bit file, change bit depth back to 24bit and deselect interleaved (if that's how it was setup beforehand)....