Copying the full filesystem should've worked for OS X too, though not copying merely the contents (i.e., you need FS-level metadata). But yes, from OS X you ne

Copying the full filesystem should've worked for OS X too, though not copying merely the contents (i.e., you need FS-level metadata). But yes, from OS X you needed a secondary boot loader (BootX) to be discovered by OpenFirmware, whereas for classic Mac OS OpenFirmware itself found the payload to boot (which differed between Old World and New World systems).