Hi,
I’ve got an RPi3 wit an Allo Boss, and I use a couple of the GPIO on the Boss’s J19 header (see my previous post, this thread) - one for switching my amp, and one for an IR receiver. If I understand your question correctly, you want to get access to one or more GPIO pins, but can’t because the Volt blocks the J19 header. I see two possibilities for you: 1) stack a proto HAT of some sort between the Pi and the Kali. If you use stacking headers on the proto HAT, everything should get passed straight through on to the KAli and the rest of your stack, but you’ll be free to tap unused signals at will. You can even use the I2C bus, as long as you don’t clash with the Boss’s address. This is the most flexible solution, as it will enable very easy access to ALL of the unused pins on the PI, not just the ones passed on tho the J19 on the Boss, but it will make your stack even taller than it already is (whether this is a good thing is a matter of taste, I suppose). It will also provide a convenient way to integrate other stuff you may want to add, such as circuitry for an IR receiver, connectors for hooking up buttons, encoders, etc. 2) break out your soldering iron and start hacking the hardware. This keeps your stack height from growing, but will be more technically challenging.
Good luck!
Chris