I was under the impression that SD was designed to be a superset of MMC, e.g. that if you plugged one into an MMC socket the equipment would read it in that mode.
That sounds sensible but the SD designers may not have been.
Is it that SD equipment can read both cards but MMC equipment can only read MMC?
Your project is very welcome, those configuration memories are way too expensive.
I wonder how hard it would be to have a small micro (e.g. AVR 2313?) reading a FAT-formatted SD card to get a fixed named file (e.g. FPGA_CFG.ROM) to load the FPGA?
That would allow ordinary PC software to just copy the file across, rather than have to write to a linear sequence of blocks.
As a useful bonus, the SD card after FPGA-loading might be handed over to the target micro for normal memory card duties. So long as it doesn't mess around with the FPGA_CFG.ROM file of course!
The target micro may even be the same as the loader micro.
I hear FAT handling is non-trivial, so perhaps having written it for the loader micro the loader could also be useful as an I/O slave. Might save a lot of porting work.