A typical Atari ST IDE board (which would be the same hardware, essentially) is four (or less) ICs on a few square inches of double-sided circuit board. That's not exactly expensive considering the price of SCSI-IDE bridges. It is, however, a lot of a hacking.
(If you're willing to replace the stock Mac ROM with EPROMs containing a hacked version my guess would be the simplest way to do a driver from scratch would be to start with the Mac Plus ROMs and replace the SCSI driver with your IDE drive code. The only thing the Hyperdrive source would give you is some hints on how to make the whole lashup work with the 64k ROMs. Hyperdrives worked with those, right?)