I finally got around to trying the lepton on a Pi Zero.  Not a good outcome but an interesting result.  The interrupt-driven version of leptonic failed miserably.  It could not keep up with the VoSPI data.  However Damien's original version did work partially.  It could get data but constantly lost sync and frequently returned garbled data.  I didn't look at the SPI bus on a scope to make sure there was nothing funny going on there but it doesn't seem the Pi Zero is well matched - at least for user-space programs - with the Lepton 3.  No doubt a kernel driver would work but that's beyond me at the moment.

I have also been hacking around at the application level to make sure that the cross-platform application development tool I use (xojo) is up to the task and could be integrated with the zeromq messaging library since it makes working with sockets very simple.  Much more success there.  Here's a test app running on my Mac and also on a Beaglebone Black simultaneously getting a feed from the leptonic server on the Raspberry Pi 3. Getting the full 9 fps from the lepton.  I think I'm ready to try to use the PRUs as a VoSPI engine.