Basically, this boils down to the C65 DOS ROM being able to use the FDC, without getting stuck in any of several infinite-loops that result if the FDC does not behave as expected.
So now, if the SD controller is told provide the FDC with a disk image, and the C65 boots, the drive light flashes briefly, before loading of the boot file fails (because right now the FDC can't actually read sector contents, and the image I am providing is a placebo piece of SD card memory, not initialised with a 1581 disk image).
All this can be easily detected on a C65, because the border colour increments each time there is a disk error. This was presumably a diagnostic feature that they intended to remove once they were happy with the ROM, but just as it was useful for them, so it is useful for me. So here you can see the C65 starting up with the border incremented from blue to yellow:
(Note that in 80-column mode on the C65GS there are no side borders by default, since 640x3 = 1,920 pixels. It would be possible to have side borders, but using this resolution they would be 320 pixels wide each, which would look silly.)
So I decided to push things a little further, by talking to the DOS using the C65 ML monitor for convenience. The ML monitor can be entered at reset/power on by holding down run-stop during reset.
It was nice to see that I could ask the DOS for its status. I then tried $ to get a directory, but the C65 monitor thought I wanted to convert a hex value to decimal. So then I tried to load a (non-existent) file into memory. That produced an I/O Error #4, presumably because when it tried to read the disk header sector, it read all $FF bytes (which is about all the floppy controller can return at present), and so gave an illegal track or sector error.
But it is still very pleasing that it can do that much.
It shouldn't be too hard from here to get it actually reading sectors from the SD card, once I figure out the way the FDC buffer operates.
In preparation I will modify the kickstart ROM to look for C65GS.D81 on the SD card, and pointing the FDC to that by default. I will have to check that the file is contiguous on the SD card, since the FDC emulation assumes continuous sectors. I will also need to make sure that it can't write beyond the end of the disk image and corrupt things.