I've finally managed to advertise shares using the dns-sd executable (with identical results on Mountain Lion and Windows). I can force any of the default icons to show up in Finder with a command like:
dns-sd -R ComputerName _device-info._tcp local 445 model=AppleTV
dns-sd -R ComputerName _device-info._tcp local 445 model=Xserve
I've even come pretty close to writing the bat scripts for windows to automate this.
But I can't figure out how to register new icons. I did make a copy of MobileDevices.bundle, and populate it with new icns files and entries in the plist. But there is no indication that the system recognizes the file is there.... and I'm hesitant to alter the originals. Is it enough that the bundle is placed in CoreTypes.bundle, or does it have to be registered some way that I'm not realizing?