Arduino as SPI slave with self-triggered SS?

I'm trying to fix a control device(programmer) for an old piece of AV test equipment (henceforth referred to as Keyingdevice). It works with three communication lines (0-5v) - ready out, clock, and data out like this:

  • A potentiometer is turned on programmer, so this information needs to be conveyed to the Keyingdevice
  • Programmer sends ready line high (5v)to Keyingdevice
  • Keyingdevice sends 8 clock bits
  • On each rising clock pulse, the programmer sets data line high (5v) or low (0v) depending on the value (expressed as a byte) that it is trying to represent
  • Programmer sets ready line low to close communication

All of this happens within about 700μs.

I've found the standard Arduino commands too slow to work with this. I'm told I should use the Arduino as an SPI slave. Usually the slave has to be selected by setting SS high. In this use case the slave sets the ready line high itself. Could I route a digital out pin from the Arduino into it's own SS pin to trigger it?

Any examples of something similar?

SS is active low...