Re: I think it's a reasonable idea.
I think this is a reasonable idea. I seriously doubt anyone who nicks a device goes in and spoofs the MAC address.
1 - law of unintended consequences. This creates a location log of anything with a MAC address - provided they get into your home WiFi network which is where I'd raise red flag no1: is that part of planned capacity. This also creates a log of where MAC addresses may live which nice feeds into this Big Data fetish they have where more data is presumed to represent more accuracy (newsflash: not if you feed it garbage to begin with).
2 - "evidence". First of all, who knows the MAC address of their kit, so how can they provide it when it's stolen? Where does that data come from and how reliable is it. And if it is reliable, how did it get to the police without you being involved? Secondly, flipping a MAC address is networking 101, also for very normal, acceptable reasons like testing.
3 - processing. What is the police going to do when they find a MAC address (which, for sensible reasons, suggests they have hacked the network it lives on)? Go in, in full SWAT mode? I suspect someone is already coding up malware to distribute flagged MACs into home user kit, so this really becomes another exercise in handing back tax payers some of their money through court cases. That is, the ones that survive the experience.
So no, I don't think it's a good idea. Not even superficially.