Hmm, I thought that ext3 was virtually identical to ext2 but with journalling capabilities...which wouldn't be used when mounted ro. Have you tried mounting the ext3 partition ro as as ext2 parition?

From http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html



ext3 filesystems may still be mounted by ext2 as long as they have been cleanly unmounted. ext2 will refuse to mount ext3 filesystems which have not been cleanly shut down, because there is live data still in the journal which ext2 does not know how to deal with.

The e2fsck application from e2fsprogs can perform journal replay, so running
e2fsck -fy /dev/hdXX on a damaged ext3 filesystem will repair it, allowing ext2 to mount it.

ext3 software will refuse to mount an ext2 filesystem - at present there must be a journal file on the filesystem.