You can mount it with hdiutil
 

I had this problem with a udf image I was playing with (that I'd made with a recent beta of mkisofs). hdiutil, for some reason, is picky about the file extension. It wouldn't have anything to do with my image *.udf, however, when I changed it to *.dmg (just a name change, nothing else) it mounted the image fine with the command:

hdiutil attach *.dmg

btw, after some experimentation, I was able to mount the image using the hdid -nomount *.img command followed by the mount command only after I'd created a directory which shared the volume name of the image being mounted. This doesn't seem terribly useful, but there you have it.

how did you mount the ISO using the mount command?

well it worked this morning :(
 

So I had a UDF image lying around whose volume name was 'REVOLUTION_OS'. I assigned it a block device like so:

hdid -nomount RevOS.img

Then I made a directory:

mkdir /Volumes/REVOLUTION_OS

Then I issued the mount command:

mount -t udf /dev/disk2 /Volumes/REVOLUTION_OS

et voila! it mounted.

That, however, was this morning. The algorithm doesn't seem to work now.

btw, I tested this out also by making an iso9660 with mkisofs called downloads.iso. hdiutil would mount it fine with the command:

hdiutil attach downloads.iso

However, a similar algorithm to the above doesn't seem to work.