I finally got it!
I followed an awkward way but it worked. In short, I installed windows 7 and vmware and used a Leopard vmware image (http://www.sysprobs.com/install-mac-os-1055-leopard-virtualbox-314-installation-dvd ) to dump the premade image to the pendrive. It worked like a charm!
First I tried with virtualbox (as suggested in the link), but bzcat was taking ages. When using -vv to get some kind of progress information, it was going extremely slow.
It's a pity that I didn't manage to do it with Ubuntu instead. In fact, I didn't fiddled with disk utility nor something distinct than unmount & bzcat.