Yes, it's possible. I figured this out almost accidentally while doing some hardware maintenance tonight, so I thought it'd be worth sharing :-)
This may only apply to the vertical drive bays. A 5.25" -> 3.5" mounting tray (also known as a drive sled, still quite common at regular computer stores) can mount directly in the rear bay from the underside. If you use mounting brackets instead of the tray, you'll find that the gap between them will still be too wide to hold a hard drive. You could probably get around this with some longer-than-normal screws and some bolts.
For the front bay, you'll also need to use some bolts to raise the drive tray to the same level as the rear (which is raised slightly due to the design of the assembly).
The same mounting holes also exist on the bottom horizontal bay, but you'll need stacks of bolts in order to reach a CD drive mounted there. A hard drive might be easier. It also requires major surgury to access that part of the tower.
Hopefully this helps some people who've been searching for drive rails.
