I agree that this is a potentially neat project.
What are the design parameters, and how many of them are overcome by use of a milling machine.
1: Lots and Lots of grooves to be milled. I have read that bats like horizontal grooves an eighth to a quarter deep and as close as a quarter inch apart.
2: May be single layer thick. Imagine a box with dimensions x = 12" y = 2" ( one inch if I remember right for the internal cavity, and assuming 1/2 inch material.
3: If multiple chambers, then each chamber is 1" and the dimensions change with the # of layers. and the thickness of material chosen.
4: sides are not an issue. Not much stress, so joinery is a secondary issue. Of course if you were designing for a box built by a CNC machine, you might build the internal panels into the sides as a strength enhancement.
5: Holes between the layers or chambers of the house are easily built with a CNC machine.
6: After building, you have to mount this thing 10 to 15 feet high, facing south, and not necessarily on a tree, as Bats don't like to fly around the limbs of the tree. A building is best, I believe, but a stand alone telephone or other type pole is ok.
That's what I remember from reading several sources, and it sounds like NathanButler has some pretty good knowledge and spelled out several things that I didn't pick up from what I've read.
The X-Carve is a really nice answer to a lot of the issues of the project.
Advantages of using an X-Carve:
A design could be made and improved on by the community, and everyone wouldn't be starting from scratch on their own efforts.
The work itself would be easier than trying to accomplish with a table saw or router table (I've tried both, and the simple movement of fences and carriages is a lot of work)
Not overcome by X-Carve:
You still have to hang this thing higher than most people like to go on a ladder, assuming that they have an appropriate place to put it, and that you give a hoot (that's an owl joke) about bats anyway.
Overall I give this idea a thumbs up, and it's something that is on my list of 'stuff to do' in any case, whether someone likes it or not.
Lastly, it may take several years for a bat to move in, so this is not instant gratification.