I've done a few mirrors with parking sensors, don't really need radar detectors in the UK... I don't think the Police can use them over here because they're not efficient enough, think we just have laser guns instead, but devices to jam them is illegal (some people get around this with some sort of front parking sensor from memory)... Could be wrong!
Anyway I just used mirrored tint film on bare glass, any light behind the glass will shine through like the pic of the M6 mirror on this thread, or if you really want this 'etched' look then you can still use film but get the supplier to cut it however you want.
Also if you're putting something like this in your car, wouldn't you want it hidden? I know in the EU you'll get fined for having such a device regardless of your speed!