Try to get it booted up (slowly) without any partition managers, i have found them to be quite unbearable and ready to create havoc, although a reinstall of OpenBSD does not take that long.
If you don't mind waiting a bit longer to boot, stick with the regular EFI that comes with your Macbook, if your feeling brave go ahead and try out some boot managers, plz post any success or horror stories i would be interested in the outcome.
The MacPPC build will not work with the newer intel cpu/chipset Macbooks, give a try to the i386 and amd64 builds im sure you will like