There is a thread that discusses using IsoBuster to create an image. You need to back up the entire hybrid disk (at least for 10.6).
Can you access another Mac to do this? If so, in Disk Utility select the drive (not the volume) and click the New Image button on the toolbar to save a DMG as a DVD/CD Master. Funny idea: go to an Apple Store with an 8 GB or higher USB stick, buy Snow Leopard, then use one of the computers there to create the DMG.
Once you have the DMG, rather than using diskutil at the command line, just use Disk Utility and create a 10 GB partition at the end of your drive (under the Partition tab for your drive). Then go to the restore tab and select the DMG for the source and the new partition for the destination. Make sure Erase destination is checked and double-check that you're dealing with the right partition.
Note that I'm using Disk Utility on 10.6 and I haven't worked with 10.4 in a long while. My thinking is that Disk Utility hasn't changed very much and I hope that the features that I'm talking about exist and behave as I would expect on 10.4.
Finally to install Snow Leopard, reboot your computer, hold down the ⌥ key as soon as it starts up (chime) and you should be presented with two options, both with a hard drive icon. The second option should say Mac OS X Install DVD. Select that.
Specific IsoBuster instructions from the above thread:
Open IsoBuster -> right click "DVD" on the left side -> choose "extract dvd " -> User Data (*.tao, *.iso)
Name the file whatever you want (ie "Leopard") on the "Save as type" choose ".iso"
Click Save and wait for image to be created. After it is finished it will probably ask you to save a ".cue" file as well...go ahead and save it, but you should not need it.