Sounds like a great idea. Taking this one step further, would it not be possible to make it a full length card and have 2 HD's one the same mounting card (perhaps it need some metal rail on the top for stability reason)?
And why stop there? Why not add a 25pin external plug and a 50pin internal connector for SCSI to the mounting card? Hmmm... perhaps I got a little carried away there? :-D