It's probably not a good idea because of heat, etc. But... being a former Shuttle owner I have seen some pretty creative stuff. Depends on what model you have, but I'd start thinking along the lines of a vertical mount.
Edit : You just have to do a lot of brainstorming and visualizing for this. When I installed a water cooling setup in my shuttle, I was faced with a similar problem. Some thoughts:
-Try suspending it under your HDD/FDD cage. That would certainly require some airflow modifications though.
-Remove the stock HDD/FDD mounting equipment all together. Then you could rig up your own device that fits your needs.
-Look into an ultra slim optical drive, such as
this one. That leaves a lot more room to work with.
A back-up plan is to just mount one of them externally (ie on top, on the side). That would come in handy if you ever want to transport large amounts of data. Whatever you do, keep temperatures and airflow at the front of your mind, too.
Edit2: One last thing. Check out the
SFF Tech forums for advice from tons of shuttle modders.
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