Oh my god! I was just about to post similar findings, what a coincidence!
I just had compiled a kernel with yaffs2 support. My idea was to modify /init.rc in the kernel to mount the partitions with yaffs2 instead of rfs. Problem: we can't just change the filesystem without backing up and restoring all the files inside. My idea was to do that inside the recovery. Mount a mountpoint, backup with tar to /sdcard, unmount, format with dd (in theory a partition filled with 0's is ok for yaffs2), mount again with yaffs2 and restore from tar.
I'm stuck at the point that I can't seem able to mount any partition with yaffs2 previously formatted (with /system/xbin/busybox dd if=/dev/zero of=/dev/block/stl10). If I mount (/system/xbin/busybox mount -t yaffs2 /dev/block/stl10 /dbdata) there is always an "Invalid argument" error. It happens with /dev/block/stl*, /dev/block/bml* and /dev/block/mmcblk*
I'm no kernel expert, neither I know a lot about yaffs2, but I think I read you can't mount a yaffs2 partition anywhere, it has to be an MTD flash device.
Anyway, I'm going to try with the tools you said. Wish me luck.