Tattoo Focuses have been around for a while (lore wise, the Ink masters of the Triads gave them out). But we never had rules for personal tattoo foci..
You are right that a MAD scanner would be worthless.... but it could be worthless on a lot of other foci too.... my Power Foci is actually a wooden Celtic cross carved from English White Oak... no metal to set of off a MAD there

And just like any other magic, a high back ground count does affect it
(which is one of the reasons going after a Toxic in his domain is near suicide! The background count fries your own magical support while his is at full strength, if not augmented!)
As for law enforcement... well, the only way they could control something like this would be to disenchant it.... which means kiss that Foci, cash and karma good-bye

The other options are.... a little less forgiving.