I've got a heap of DVD ISOs I don't really want to burn; instead I'd like to mount them (using the standard loop device trick) and fool Mplayer, Kaffeine etc into playing them.

Does anyone know if this is possible, and if so how to do it?

For mplayer / vlc / xine you don't even need to mount them. They play ISOs just fine.

Yeah I use mplayer to directly play ISOs.

Just for informational purposes though, to mount an iso:

mount -o loop /path/to/the.iso /path/to/mount/folder

Well, I'll be! You're right! Didn't change the file filter setting in Mplayer, so couldn't see the ISOs in the list.

Many thanks.

For the record, the command line to use:
Most of the big media players have syntax for playing iso images. xine does it in a way similar to mplayer, for example, but for some reason it won't accept relative paths. Only the full path will work.

One trick around that is use readlink to translate relative paths into full paths when passing the argument, like this:
By the way, as with the mplayer command above, the majority of programs will treat an iso disk image the same as if it were a drive with the disk inserted. So a quick&dirty method for using them is simply to go into the drive settings and replace the /dev/dvd or whatever with the path to the image.