I've discovered that it is effectively read-only anyway. Any time I try to do anything to a file in the mounted rootfs.jffs2 I get an error along the lines of "no more space in file system." I get this error even if I try to delete a file.
Also, the access speeds in the mounted image file are terribly slow. A simple "ls" takes a few seconds.
It was interesting as an experiment, and I now know that it can be done on the tablet, but not useful for running applications. It looks like ext2/ext3 are still the filesystems of choice for that.