And I did make some more progress. I got my circuit running, using a rewrite/recompile of the 65SPI/B code. I can initialize the SPI environment, I can read/write a byte and even the (basic) SD implementation is working. This is largely based on the driver by André Fachat, but rewritten for the Apple ][.
The only issue I now face, is to actually make the SD card accessible from within ProDOS or GS/OS. My goal is to have the card bootable from the SD card, perhaps support APM (Apple Partition Map) partitioning, so we can also use HFS partitions as the SD cards are getting larger.
Currently the code I wrote supports SDSC and SDHC cards (SDXC isn't supported?), but I cannot (yet) use the SD card from within ProDOS nor GS/OS as I haven't.