I found this project by Bruce Abbott:
Testing with a stock A500 gives R/W speeds of 64K per second (over 114K with a A1200/030). It uses a system friendly device driver and has to wait on the Zip drive to respond to commands. So, I should be able to beat this speed with a DMA'd SD card. That would be plenty fast enough for copying files.
I guess if the partition was setup with CrossDosFileSystem I could probably just make a block transfer driver without having to handle the FAT32 on my device, but I bet that would really kill the speed. CrossDos is pretty slow,