After much VHDL and SD card protocol wrangling, and the odd spot of FAT file system implementation, I finally have the C65GS able to load the main ROM from SD card when it boots, and then automatically switch to C64 mode. As can be seen in the video, loading the 128KB ROM file is very fast, somewhere in the hundreds of kilobytes per second on the boring 1x 2GB microSD card I am using.
<zort! this video cannot be displayed>
Next steps will be to refine this a little, and then get it booting the C65 ROM. Pretty much all of the 4502 CPU is there and working (the bootstrap ROM uses a lot of 4502 instructions, including the MAP instruction that will be vital for C65 ROMs), so I am hoping that it won't be too hard.