CDFS 2.61+ supports the following combinations of logical format.

 ISO9660
   ISO9660 names 
   ISO9660 names with ARCHIMEDES system extension
   ISO9660 names with RockRidge system extension (only 'NM' used)
   ISO9660 names with Joliet extensions 
   ISO9660 names with Joliet extensions with ARCHIMEDES attributes

 CD-I
   ISO9660 names 
   [ISO9660 names with ARCHIMEDES system extension]
   [ISO9660 names with RockRidge system extension (only 'NM' used)]

 HISIERRA
-   ISO9660 names 
   [ISO9660 names with ARCHIMEDES system extension]
   [ISO9660 names with RockRidge system extension (only 'NM' used)] 

Bracketted [] items are unlikely to exist in reality, but in theory CDFS will
process them. For example a High Sierra CD-ROM with ARCHIMEDES attributes
would make use of that RISC OS information, or where RockRidge names are
present these would take precedence over the base ISO9660 name. It's not 
possible for the ARCHIMEDES and RockRidge extensions to coexist as they're
both using the system extension area.

One option would be to have a system variable to follow DeviceFS' lead 
which controls the mount options, allowing a preference to be made selecting
from a CD-ROM which has more than one of the above extensions in use.
Linux has a "nojoliet" and "norock" switch to the mount command, so something
like

  *SET CDFS$0$Options nojoliet;norock;noacorn

would force the mount to use only the ISO9660 names on drive 0, and

  *SET CDFS$0$Options norock;nojoliet

would use the ISO9660 names but also consider the ARCHIMEDES system extension
if present.
