If you have fuse installed, then there's fuseiso (most distros have it packaged) or mountlo
(http://lkml.org/lkml/2006/2/27/148), which can mount arbitrary file system types.
Kernel based loop mounting would be useful, but first each filesystem's code must be audited,
to make sure they handle any filesystem image, even one specially crafted with malicious
intent. This is a big job, it is much simpler to use some completely userspace solution, like