If you replace the Optical with a HDD, I don't see why that wouldn't work. Also, if you could find a way to mount and power a second HDD, you could run it off the same IDE port as the first HDD if you get an IDE cable with a second plug on it but this is not ideal at all. Best way to have two would be to replace the Optical I would say.
[doublepost=1486666223][/doublepost]Also, I see you have a 20" that needs a logic board. Ever consider turning it into a monitor instead? I have wanted to do that to my 20" iMac for a while. I want the iMac to appear to still be complete, but it would just be a monitor/USB3 Hub/USB hard drive and optical, all internal to the dome still. Just a DVI port on the rear with a row of USB 3.0 ports to use, and one power cable and USB cable out. Then a HDD and Optical inside would be converted to USB and plugged into the hub, inside the dome. Basically, you would just leave it on your desk and connect it to your PC/Mac under the desk. This, IMO, is better than building one with a NUC inside for example, because you can use it with ANY computer, not just the one inside.