First up, Nubus is an intelligent protocol and it requires a Nubus controller IC on each card. So you've either got to find those (hah!) or reverse engineer one, then build something to emulate it. It's hardly a "throw together", and there's been plenty of discussion around these parts as to why not.
Second, can these ACARD chips be bought on the open market as bare chips, in small enough quantities and cheap enough to make it viable? IE, quite a lot cheaper than buying the complete SCSI-IDE converter
Thirdly are you sure it's just a single chip and not a combination of chips on the SCSIDE boards that do the magic, including say a proprietary ASIC that ACARD is unlikely to want to sell or just hand out the design? Seeing as they have this profitable line in converter cards we want to undercut.
All of the above being good reasons to go for a fresh PDS design. Yes, I agree it would be best if it -pretended- to be a known SCSI device so we can use existing drivers. How about the fast SCSI PDS controller from the WGS95?