You might need to patch the OSInstall.mpkg inside the installation folder of the disk.
Originally Posted by Zellio
Open the package, and edit OSInstall.dist file with text editor.
Find this entry:
---but g4 machine is not there---
var badMachines = ['iMac','PowerBook1,1','PowerBook2,1', 'AAPL,Gossamer', 'AAPL,PowerMac G3', 'AAPL,PowerBook1998', 'AAPL,PowerBook1999'];
find your machine and delete that entry (or delete all ,if you want) inside the square bracket. While there you might want to check the whole code entry and edit it because it also check for bootRom to install so you might want to adjust it too..(again, up to you)..
When you done editing save the file OSInstall.dist , put it back to OSInstall.mpkg.Make a new disk image with that patched file to install.