Have you tried the old trick for mounting iso images -
mount -t jffs2 -o loop <sheeva_ubuntu_rootfs_file>.jffs2 /media/<some directory>
Though I do see a couple of things that say this won't work. There seems to be a more in-depth way to go about it too:
http://wiki.buici.com/wiki/Mounting_JFFS2_Filesystem_in_RAM