MOS-HD:

On DH0: (first partition): Is there the Kernel for G5 (64-Bit-type) located and named boot.img?
What is your Morphos-boot-partition? Morphos or Morphos2 ? Which one is set to bootable and to what priority?

If kernel is copied to DH0: and this HD is fittet to the first SATA-Channel of the G5, try to boot from OF:

mac-boot hd:\boot.img BD=MorphOS (or MorpOS2 -wherever your MorpOS-System is located)

Make sure that the right kernel is on DH0: It will be found on MOS-ISO in drawer mac-ppc64.
Then make sure that your MorphOS-System-partition is marked bootable. Do not mount the 2nd HD to your MAC during this tests.

If this works, we can see further what to do.
Btw: I don't think that this is a hardware-issue

MACOS:
OF is searching for a so called "tbxi"-signed file on all partitions, which is in fact a bootloader. OF should find that file on your Mac-partition, which seems to be the 5th on your second HD. (btw.: MOS uses the same mechanism for autoboot).
Maybe there went something wrong with MAC-bootloader and MOS-bootloader. Lets see first what happens if yo try to boot MOS manually as discribed above.