You may have gone into detail about this elsewhere, but lets take a look at that ... I presume it does not require a special software driver?
The only way any drive can boot a 64K ROM is if it imitates one of the drivers coded into the ROM, or patches its own code into the ROM early on, which means there is some kind of ROM on the HyperDrive controller just like the MonsterMac board. So no matter what, software is being patched into the 64K ROM when it first loads into RAM. So I am back to the 64K ROMs are being updated in order to use any drive as bootable. The question remains, how extensive are the changes being made to 64K ROM so as to maintain their "authentic" functionality?
As for being HFS compliant, wouldn't that be up to the System? I mean the HD20 loads the HFS code into RAM as the system boots. To conserve resources, a bootable drive would only need to patch ROMs to make it driver aware. The MonsterMac board may include the HFS resources as a System INIT just like the HD20 does. I would think the same would be true of the HyperDrive. Why couldn't it be partitioned as HFS? That's simply a matter of the disk formatting utility which runs after the System loads.
Then again, I am merely speculating as I have no real idea how the whole boot process works.