Arbilist,
Add 6 inches to the room and that's the wall length my mill is sitting on. But for various reasons I'm locked into only one wall my 3/4 sized Bridgeport clone can be located on. I do have an entry door that does line up with my mills table so that's very helpful. If you don't locate your mill centered on that wall you will sooner or later run into a very long part that can't be located on the table to fit how you have it arranged. Yes centering it you'll also have the same, but nothing is ever perfect or ever big enough.
I'm also a huge user of fly cutters, and for the home user that can afford to take the extra cutting time, there a very cheap alternative to end mills for sure. Since I don't yet have anything to sharpen end mills, I'll take a fly cutter every time unless there's a real definate need for an end mill. Slots etc. I've got a variable radius fly cutter that takes both triangular and round carbide tips that's the very best I've ever used. I actually doubt you can do what you'd like without going into something very much like a full CNC enclosure. The CNC Zone forum would be one place to start, But if you can come up with multiple hinged panels that can also be set for vertical height? That will keep most of the swarf where you want it. If your using flood coolant? That really complicates things. Personally I'd build it free standing and not attached to the mill at all. A lathe is pretty easy in comparison to a mill for swarf and coolant control.
But everyone is of course different. You may work on a general range of long parts that might be worth offsetting your mill. But to state the obvious? At less than 54" for any long parts you'd generally have, I'd just center the mill to the room. If it's a non concrete floor, I might shift it one way or another just to match up with the floor joist's. I had to do that.
Pete