Just now reading more fine print in the ATTiny84 spec. I think I can only do SPI slave in modes 0 or 1 using the USI module.
Seems that way after reading AVR319 too.
It's not a show stopper, but the SPI master is also communicating with a device using mode 2. So, I'll have to dynamically change the SPI on the master device (rpi), when I want it to communicate with my Attiny84.
Just looking for a confirmation of that--(or not).