Another is to use tar or cpio to create the image, then run
split to cut it up into CD sized images, and convert each
piece independently to ISOs and burn them. This can, of
course, be done even if one uses compression.
Also, one doesn't necessarily have to back up everything at
one time. It might be possible to back up different parts of
the directory tree.
You could do a complete backup using tar or cpio, then split
the archive, and burn CDs. Will take about 3 or 4. Then on a regular
basis just backup what has changed. This hopefully won't require
making a tar or cpio archive larger than a CD.