You can make a yaffs2 image from the ubuntu rootfs but you cannot burn it. The image produced is 2.1GB, superboot210.bin cannot burn an image > 2GB, if you have a tiny210 it just won't fit, if you have a 2GB mini210S it won't fit, if you have 1GB mini210s, it won't fit. If you have a 4GB mini210s it will fit but superboot can't burn > 2GB.
For people with 1GB or 2GB boards I would look at ubuntu-core (like debootstrap but for ubuntu) and build your own image from scratch. Just grab the kernel/modules from tar balls.
for people with 4GB boards, my only suggestion is to use ubuntu on SD card, then apt-get mtd-utils, then from there you should be able to burn the image to nand or you should be able to format the /dev/mtd4 device, mount it and then untar the rootfs to the partition.
