The install packages contain a line in /Contents/Resources/???? (can not remember the file name) that specifies MachineID = "<NUMBER>" that corresponds to the model of computer that it is capable of installing onto (like G4 processors only, or Powerbook G4 17" 5,9)
The install discs come with the restriction stating exactly which model of system it is capable of being installed on down to the hardware revision.
Sure, one could always copy the install disc to your computer and edit every single *.pkg file so that the MachineID is correct, or you can just go get an actual install disc without the MachineID restriction.
Or, you could use an external hard disk, copy the install disc to it along with /usr/bin/open and Pacifist (avaliable from
www.charlessoft.com).

Caution, this is a hypothetical situation that I have not tested but it MAY work
Then, you should be able to install by doing the following...
Format your internal hard disk through Disk Utility
Open Terminal
/Volumes/External/open /Volumes/External/Pacifist.app
In Pacifist, select the Mac OS X.pkg file and choose install.
I have no idea whether this will work for installing an entire OS, but I have done simmilar steps in the past to set up dual booting between OS X and Linux (using VolumeWorks on an external hard disk along with /usr/bin/open to allow me to resize my main internal hard disk and in turn leave everything intact).