Hi Kris,
I cloned the MBP to an external Firewire drive, which was also formatted with the APM. Then, just like you did, while booted from Tiger on the G4, cloned that external drive partition to the SATA drive in the G4. All drives involved are formatted APM. Then I booted from the cloned partition on the SATA drive, and viola, was running Leopard on the G4.
After I ran Software Update to try to get to 10.5.8 on the cloned partition, the G4 kept rebooting over and over and over instead of the usual double boot. Went on for an hour. That's when I powered down, unplugged the SATA drive and rebooted from a Tiger partition backup on an internal ATA backup drive. The SATA drive has 3 partitions, one for storage, and a Tiger partition and now the cloned Leopard partition.
In retrospect, I don't know what I was thinking using Smart Update on the Leopard partition on the SATA drive in the G4 from the external MBP backup... just letting SD do it's thing I guess, but there is, of course, no way to smart update a 10.5.8 drive from a 10.5.7 drive
I was attempting to restore the original cloning I had done to the SATA drive, but apparently didn't have my thinking cap on at that moment!
So, using the Restore All Files in SD wiped the drive clean and re-cloned the Leopard partition to the SATA drive in the G4. And again, I did that booted from Tiger, going from the external Firewire drive to the SATA drive in the G4.
So, maybe I'll try the Combo updater and see if that helps.
Thanks as usual!
Chris K