I thought you could select the format in Disk Utilities, but I couldn't find any way to do that. I eventually ripped the image off the DVD in Disk Utilities (creating a new image from the DVD), and converted it using the command line util hdiutil:
hdiutil makehybrid -o filename.iso -joliet -iso filename.dmg
This command converts the HFS+ format image (filename.dmg) to an ISO9660 image (named filename.iso), which can then be burned using Disk Utility.

