I had the problem of trying to play my disc images without mounting or burning them.

  1. I have switched to the xine backend for totem etc to get working subtitles/menus
  2. xine supports this type of argument: dvd:/[path to dvd file]
    e.g. dvd://home/username/dvdimage.iso
  3. using the nautilus right-click menu you can "Open with" and put in a custom command such as:
    totem dvd:/%f
    And it works!!
    xine dvd:/%f
    would work too if you like xine (I prefer totem)

also .nrg (nero burning ROM's iso-like image format) is similar to ISO in a lot of cases will simply work as one.

so I used a package called assoGiate
sudo apt-get install assogiate
"File Types Editor" to alter the "raw CD image" type to include *.nrg

That is like magic, as soon as you change it and hit refresh in nautilus your nrg files appear as CD images.

thought I would share my experiences...