NetBoot,
forget that "00000000-0000-1000-8000-<MyMacAddress>" stuff; that's obsolete!
Chameleon gets the real uuid from dmi bios. If that fails, a default uuid is used. What happens next is that the real uuid is converted to a new one, as Gringo already pointed you to the explanation. You can still check the real uuid to see if it's correct:
- it's posted by Chameleon while booting with -v; you can also check this same info from the "new" booter log with bdmesg tool.
- can also be checked on ioreg under "IODeviceTree:/efi/platform/system-id".
As you mentioned, the Legacy kernel also has a generic uuid, but that is only used if no uuid is already published by the booter.
If all this fails, then you can use the SystemId key, but use the real uuid, not the MacAddress.
Hope this helps