I don't know the answer to the following question, which is why I ask it: does that happen if you use 9660srv
under Plan 9 (-ish) to access those images? (Perhaps you already are, but it would still be useful to know.)
It's quite a long time ago, but as I recall early CD formats took a literal view of permissions: "I'm on a read-only medium,
otherwise you'd be using the later and probably incompatible standard for RW optical, so all my file system permissions are "read".
Later people took advantage of having umptieen parallel trees in there in different formats (Joliet for MS, High Sierra for UNIX-ish)
and Plan 9 added its own. I think those early CDs relied on that. Later ones added parallel structures in both Joliet and High Sierra
format so you could get more accurate meta-date out of the CDs when mounted on Windows or SunOS.