I haven't personally done this but I'm pretty
sure it
was done by one of my friends. I even have some weird
"hybrid file system" hacks in RSTSFLX to let you create
a CD-R that's partly ISO and partly RTST, and bootable.
That seems quite reasonable. The ISO9660 standard leaves
enough blocks early on undefined that you can build a
CD that is both a legal ISO9660 filesystem and a legal
OpenVMS (ODS-2) filesystem.
If you try hard enough, you can build a hybrid system where
the ODS-2 and ISO9660 sides have different trees but share
the same data (useful for stream-LF and binary files).
I presume RSTS is ODS-1, which IIRC is ODS-2 but without
a few of the bells and whistles so the structure is
quite close to ODS-2. I would expect that you could
interleave ODS-1 and ISO9660 fairly easily.
Antonio