I did exactly that for my watercooled mATX LAN rig.
The first effort had the rad (old Swiftech dual 120mm fan rad) mounted to the side panel, which for the case I used (old IBM Aptiva mini system) wrapped around the top and the other side as well. Very annoying when mucking around in the case: had to unscrew the rad and then remove the piece. Assembly was worse.
The second effort utilized some of what I learned from the original-
hole was slightly resized to allow the rad to be mounted to the case frame- cover slid OVER it. This was much better, but still made accessing things tough.
The final (current) effort adds upon that:
rad is the same- mounted to case with a hole in the panel.
Motherboard, pci/agp cards and water pump are ALL on a removable tray now.
To access stuff:
remove cover
unbolt rad
remove one screw holding tray
carefully slide radiator and tray out of the case.
I apologize for taking things further than the original topic but I though a few of the things I learned might be useful to others who are considering doing something similar, although I'll admit I ended up deciding aginst a window after all.
I had one started when I was first building the rig but ended up deciding that it was so cramped inside that it wasn't worth looking at, lol.