So let me see if I have this straight.
You have a PowerPC iBook G4 and a 350mhz Powermac G4. The Powermac has no DVD drive while the iBook does. Now you want to get Leopard onto the Powermac.
Does your iBook already have Leopard installed?
If so I would connect the iBook and Powermac by firewire. Then start the Powermac in
Target Disk Mode, so that it may act as an external hard drive. Then from the booted iBook install Leopard onto the G4 which is acting like an external hard drive. Though from your posts you may have done this already to try an install.
Anyways if you can not install to the Powermacs hard drive this way. I would boot the iBook normally (not the install disc) then boot the Powermac in Target Disk Mode. Then use
Superduper! to clone your iBooks HD onto the Powermac, just use the free cloning part there is no need for buying the full featured program. Once completed the Powermac should boot normally into Leopard. I did this type of cloning before with Tiger onto an unsupported G3 iMac in theory it should work for Leopard as well.
If the Powermac does not boot properly after using Superduper! to clone your iBooks HD I would boot the Powermac into Safe Mode (hold down the shift key while booting). Then go to /System/Library/ and delete the two files extensions.kextcache and extensions.mkext plus repair permission with Disk Utility and reboot, this forces Mac OS X to rebuild its drivers list. Do not delete the folder Extensions

.
Note: This procedure will only work from one PowerPC Mac to another or one Intel Mac to another. Trying this on Intel to PowerPC or viceaversa will only result in a unbootable computer without a reinstall.