That mobo would work just fine in OS X, it actually a recommended board over at tonymacx86 and should take very minimal effort to get up and running.
You can either buy a Lian Li, Mountain Mods, Laser Hive or find some other case that can be taken apart for the motherboard tray and rear IO section. Or if you have somebody in metal working they can CnC you out a rear panel as well. I personally would buy the motherboard kit with the rear panel and then cut it to what I need. I have done a couple mods with the Mountain mods tray but they have been getting to be more money as of late. My next project with my Cube and my Mac Pro will be done with a Laser Hive set up. You can email them and they can actually make custom parts on the fly. Don't know what the full cost would be for that but I can say that is all they do, make modding kits for various Apple cases.
When you say hard drive bays what do you mean by this? The fact the you can use them to slide the HDDs into the bays or?
Yeah there is only one guy that is making those PCB boards for the hotswap set up for the Mac Pro cases, I do believe it is only for the 2009 model. Not sure if there is a difference between the 2006 and 2009 cases but I know the 2010 case is.
That hekokimushi person is selling them on some Chinese site for almost $300 US. You could download the free PCB maker and design your own and see what the proto cost will be to build if your into that kind of hobby building. I will be designing my own board for the 2010 case but I will be offering up the file once I get it done, don't know how long until that as the holiday season has got me busy lol.
Wow that's extremely generous of you! $300 seems steep for a pcb board... but will look at the pcb maker and have a look. Is there a link to the site? The case seems pretty much the same as the 2009 but a possible difference where the screws lie in the psu cage.
Have a great christmas and thanks for the replies!