Is this even possible?

I'm almost out of CD-Rs, and would like to "poke around" other distros I don't have burned.

Should be. Mount it as loop device and see what you can do. Many distros will allow for a boot floppy and then installing from a specified place such as a directory on the hard drive. So you might be able to use the boot floppy to mount the filesystem on the hard drive, then mount the iso as a loop device, then point the installer there. If nothing else, copy the files out of loop onto the hard drive, I suppose.

Then again, that pretty much requires a floppy and, then again, if you're not completely broke, CD-Rs aren't too expensive. And you can use CD-RWs, of course, and only need one - then just overwrite them with your next distro.

It most certainly is. What you'll need to do is mount the ISO image as a loopback device. This page from the Slackware book describes it, but the short answer is basically to
To illustrate, "mount /home/user/some-distro.iso /mnt/loop -o loop"

Note that your mountpoint must already be defined. Good luck with it -- J.W.

Great thanks! I'd kinda figured on it being impossible.

I won't be able to download another distro today (internet gets wonky after downloading more than 2 GB), so I'll try getting one tomorrow.

I used this command in my install script for GoblinX...

mount -t iso9660 -o ro,loop=/dev/loop1 /tmp/goblinx.iso /mnt/isopoint

I think you can only mount iso as read-only, so you will have to copy folders or unpack... In some distros this way to install will be more difficult.
For livecd is a good way to install because, using my livecd as example, the packages is already compressed into directories, so you will uncompress directories and not install each package.

Have you considered installing your distros on VMware? I do this all of the time to experiment with other/new distros and OSes. VMware is currently offering a free trail of the new beta for free download, otherwise you can download a trail version from the site.
www.vmware.com