I know. This is documented well in a lot of wikis and blogs. I’m just blogging this to find it faster the next time 
First, create a new block device:
lofiadm -a <isofile> /dev/lofi/1
Then mount it:
mount -F hsfs -o ro /dev/lofi/1 /mnt
And in a single command:
mount -F hsfs -o ro `lofiadm -a <isofile>` /mnt
