Just stick it on a flash drive, I mean unless you bought another retail disk that had 10.5.7, but I haven't even seen a retail disk above 10.5.4 before! I'm sure they are out there but my school doesn't have them and I didn't see any marked that on Amazon.
You could always just do the install, update it, then boot off the install disk, plug in an external, and make a direct mirror of your hard drive using the disk utility app, then move it to another hard drive if that is what you're looking for?
It may be possible to do the disk thing, but it wouldn't surprise me if there was some kind of security (actually i'm telling you there is one) security on the installation disk so that you can't make copies of it with it modified with an update.
I'm sure there is an illegal way to hack it, but A I don't really want to talk about that since.. that's completely illegal, and B it's so much work I don't know many people that would even bother.
-Dell Mini 9 - Refurb $269: Obsidian Black, 32GB STEC SSD, 8GB Kingston SDHC (left from previous owner), 2GB 667Mhz RAM, 1.3MP Camera, Bluetooth, Mac OSX 10.5.7
-Dell e1705 - New $1055: ATi X1400 OC/ed 175% faster, XPS 1710 Heatsinks, XPS 1710 Body
e1705 is old, it needs not to be upgraded, but rather be replaced.