I've seen some apps that feature bluetooth detection for security purposes. I think this is right up your alley. I am leary of buying the other apps because I fear they might interfere with Keyboard Maestro -- they feature USB, bluetooth and location detection to invoke scripts.
Purpose? You can use bluetooth detection to lock/unlock the computer. With Keyboard Maestro you would have an advantage over the others since you might allow multiple bluetooth devices (multiple triggers). This would bring a new class of users.
Another feature might be location detection, but WiFi network pretty much does that now. I guess it might be nice if it didn't require connection.