As with all these projects, the tricky part would be getting the software support written. I posted a message a while back (years?) probably in one of the 68060 or Coldfire threads about the elegant way (broad outline, no actual useful instructions) to do this. I happened to have a copy of Inside Macintosh open in front of me when I wrote that post.

The Device Manager has some kind of structure of shims and pems. Or lems and pims. Or some such. SCSI Manager 4.3 is one of these things. Anyway, to add support for an IDE card, one would need to write at least one of each of these components for the operating system and figure out how to install them from the IDE card's ROM.

Or one could plan on the host machine always having SCSI Manager 4.3 available and make the IDE card look like a SCSI card. However, if SCSI Manager 4.3 isn't in the ROMs, then the IDE card devices won't be bootable.

FWB got around this, so I've read, by including a copy of SCSI Manager 4.3 in the JackHammer's firmware/ROM. Which is why it conflicts with the Daystar Turbo601 which also has SCSI Manager 4.3 in the ROMs (PM6100 ROMs, apparently). But then FWB provided a fix whereby the loading of SCSI Manager 4.3 from the JackHammer could be disabled, thus granting compatibility with the Turbo601.

Similar software issues with building a USB card.

The early Device Manager just wasn't that extensible. That's why, for example, the Quadra 950 appears to only have one Logical SCSI bus evne though it has two separate physical SCSI busses (two 53C96 chips). The hardware is there, but the firmware and OS is only written to admit the existence of a single SCSI bus.

So, anyway, getting the hardware going probably wouldn't be so hard, but getting the firmware and OS to recognize the attached hardware, that would be the trick.