kernel module.
it'd be near impossible to write directly to anything regarding the card, however, modules global symbols (including functions) are in the kernel's symbol table, so you can call functions defined in the mmc module from within your module.
Cool. From what you said, it looks like the symbol is exported... so if we were able to send this value to the CSD... .
I think it might be worth a short. Will see if I can do this over the weekend. It has been a LONG time since I wrote a kernel driver. I don't remember much at all... very rusty.