If you were *really* desperate it would be technically possible, *if* the stock bridge board didn't work with a given laptop drive, to use a "generic" Laptop ATAPI->IDE bridge board instead, but you'd have to do one of two things:
A: Install a longer 40 pin cable with a second connector on it between the hard drive and the motherboard and run the laptop drive as a slave. Some of those bridge boards have a master/slave jumper on them. Alternatively you could use an 80 pin CS cable. (Pretty sure I had that work. Granted it's been a long time.) However, I'm not 100% sure this would be a bootable solution.
(I've run a desktop DVD-ROM drive as a slave on the "main" IDE bus on a disembodied iMac motherboard before, but I can't for the life of me remember if I ever booted it.)
B: In theory you could convert the fine-pitch 50 pin connector that the stock bridgeboard uses into generic 40 pin IDE and wire up a generic bridgeboard that way. (The cable's standard IDE for the first 40 pins, then it has the audio and power leads.) People were doing that back in the 2000-aughts when it was still fashionable to repackage iMac guts into desktop cases. (Well, maybe "fashionable" is the wrong word. Back when "the resulting computer was arguably worth the effort", maybe.)
A: Install a longer 40 pin cable with a second connector on it between the hard drive and the motherboard and run the laptop drive as a slave. Some of those bridge boards have a master/slave jumper on them. Alternatively you could use an 80 pin CS cable. (Pretty sure I had that work. Granted it's been a long time.) However, I'm not 100% sure this would be a bootable solution.
(I've run a desktop DVD-ROM drive as a slave on the "main" IDE bus on a disembodied iMac motherboard before, but I can't for the life of me remember if I ever booted it.)
B: In theory you could convert the fine-pitch 50 pin connector that the stock bridgeboard uses into generic 40 pin IDE and wire up a generic bridgeboard that way. (The cable's standard IDE for the first 40 pins, then it has the audio and power leads.) People were doing that back in the 2000-aughts when it was still fashionable to repackage iMac guts into desktop cases. (Well, maybe "fashionable" is the wrong word. Back when "the resulting computer was arguably worth the effort", maybe.)