I needed to edit the initfs filesystem on my n810 wimax. Mounting the jffs2 image was only documented for older systems...here's how I did it on my RHEL5 system.
losetup /dev/loop0 initfs.jffs2
echo "/dev/loop0,128KiB" > /sys/module/block2mtd/parameters/block2mtd
That gets everything ready to mount the filesystem.
mount -t jffs2 /dev/mtdblock0 /mnt
If you don't have the mtdblock0 device file, you can make it (looking on the n810 I see that it's a block device major 31 minor 0
mknod /dev/mtdblock0 b 31 0
You can't edit the filesystem, so copy it off somewhere and make a new jffs2
cd -pr /mnt /tmp/foo
edit things in /tmp/foo >>
mkfs.jffs2 -r foo -o initfs.me.jffs2 -e 128 -l -n