I wanted to use BPv4 to program several AVR MCUs on one board. I was planning to us CS and AUX* pins to reset each MCU. However, I ran into troubles, actually just the CS pin worked.
I needed that functionality badly, so I made a patch for both BPv4 firmware 7.1 and latest avrdude. The patch makes CS, AUX, AUX2 working correctly. I don't know if it could be handy for anybody else, if so, I can make a pull request with the change on github.
I am actually thinking about getting AUX1 working too, that is, however, a little bit harder. I have seen some discussion about that on internet in past, but it looks like it has never been implemented. Or am I wrong?