Standard Deviation of quantizers has already been added to beta4 of xvidqp (don't know why i didn't add that before, seems useful...). What i plan to do next is to test and see if getting average BPP from framesizes as well as its minimum, maximum and deviation could be useful. Also expect new xvid builds to be working properly (without b-frames).
Beta1 is out and working but still far from complete...