That setup actually for practice & P.O.C.
I have successfully install retail Leopard 10.5.6 using retail Mac OS X 10.5.6 DVD on my Acer Aspire 3680. I'm using your Boot-132 usb (of course with my own kexts in the INITRD.IMG) to boot & install Leopard 10.5.6. I'm using combination of your guide & munky's guide. I create two partition on external hard disk (MBR); BOOT132USB restored to first partition & retail Mac OS X 10.5.6 DVD restored to second partition. After that, boot with external hard disk, format internal hard disk to GUID partition table & install. All is going well & I'm able to boot the retail OS X using your BOOT132USB. For final step, I use munky's guide so that I can boot using EFI partition.
Thank you for the guide & BOOT132USB. I really appreciate it.