After much experimenting, I figured out that an unusually low usb timeout setting was causing usb transfers to stall on the Raspberry Pi. After I raised that, I was able to easily load disk images! Unfortunately, Karateka does not work. During the boot sequence, Karateka does a very quick jump from track 7 to about track 27. It is cycling the Disk ][ phase lines very quickly to make this happen. Unfortunately, I don't think the Raspberry Pi Zero is currently able to sample the USB data fast enough to keep up. As a result, it is only jumping to track 15 or so. I was having a similar problem in earlier versions of a2usbdsk. I tried some tricks to speed things up, to no avail. But, good news is:
So, as a proof of concept this is looking good. I should be able to move on to step 2 and get this running on the Raspberry Pi's native SPI port.