I'm sorry I forgot to respond to this!
Came across this thread again when I was debugging some stuff.

Anyway, I've found the solution.

Modules must be loaded using insmod or similar in preinit in this order:
mmc_core mmc_block omap_hsmmc mbcache jbd ext2 ext3
(ext2 only as necessary)

mmcblk* will only show under /sys/block after a bit of time (ie: 1 second).
So, a sleep of 1 second was added in my script after loading above modules.

Then..., in my case, using various dev files under /sys/block/mmcblk*, I had some commands do following:
mknod /dev/mmcblk0 b 179 0
(where the last 2 values are obtained from dev files; ie: /sys/block/mmcblk0/dev)

That created devices in /dev for me.

I've got that all in some loop and script in my preinit script of course...

Thanks again for the help!

kh