Backing up a disk image with Time Machine I'm storing backup of my Gmail account in a disk image, to cut on number of files on the main HFS+ volume. This makes Time Machine ignore the existence of this volume, just because it's in a disk image.

Is it possible to get Time Machine to treat the disk image as a regular disk, and get Time Machine to back it up as if it were a "physical" volume?

This is on Mountain Lion 10.8.2, on late 2009 white unibody Macbook.
To back up a disk image, you back up the .dmgfile itself, instead of the virtual disk the system shows you. 

However, unless the disk image is a sparse bundle image. Time machine will have to backup the entire disk every time the disk changes.