The easy way to put a hard disk emulator in a Mac 512 is to add one of the SCSI-equipped upgrades and Mac Plus ROMs. From there, add a notebook SCSI drive like I have in mine, or use a SCSI-IDE adapter and a CF card or something like that. You can even beef up the analog board enough to run the drive without an added power supply. No hacking assembly language required.

http://web.me.com/henryspragens/stuff/Photos/Pages/Computers.html#10

It wouldn't be a HyperDrive, though, or use any of the HyperDrive parts. If you need HyperDrive source code, you probably will have to disassemble your ROMs and create your own, like we did with the Mac 128K ROMs to run our disk drives in 1984. A solid-state HyperDrive will either be very expensive or a lot of hacking.