Doesn't Harry tattoo protective sigils into his duster? 

Granted, they're not foci for projecting aggressive spells such as a blasting rod, but lets set that specific instance aside and consider alternatives beyond the protection spells Dresden uses on his Duster.

Molly's wands that she uses to, among other things, focus her one-woman-rave spell showed no sign of exploding, nor even heating.  In such circumstances, whilst tattoos may not lend themselves to great utility in all circumstances, they may have utility for some circumstances/spells and for some wizards.

I believe that tattoos could act as Foci for mental magic, protection, and other less energy intensive spells. 

As to who tattoos them into you, that's a tougher one - Harry did his own work on the Duster, but I think most people would struggle to perfectly tattoo themselves, particularly on the forearms, further particularly on your off-hand.  And it'd be a sloooooooow process without the use of modern (ie electrical) tattoo machines.

It's a nifty thought though...

Though with divergent histories, it's possible for someone to be N-fected in one universe but not the other.

^This. Any "knowledge" gained in the other world may not have a direct read across to Harry's own world.

Assuming the world itself is the same (i.e. the sky is blue, grass is green, magic and physics all works as advertised, Chicago is, well, Chicago) the only way we'll know something is different in the mirror world is when we see a known character acting out of character compared to Harry's reality.

What we won't know is whether they are acting differently because of different choices made/circumstances in the mirror realm, or because in Harry's world they're covert whereas in the mirror realm they are overt in their actions/alignment.

And as dspringer1 says, just because Michael is N-fected in the mirror world, doesn't mean he is in Harry's regular reality, any more than if he's got a broken leg in the mirror world you'd expect to see him in a cast when Harry returns to his regular time and place. (PS Michael's N-fection was a for-instance... not some crazy tinfoil hat WAG!)

Jim doesn't travel well, so it's unlikely he'll be crossing any ponds.  He went to a con in Canada a while back, but I think that's as adventurous as he plans on being.

Och well, I'll just have to try and schedule another work-related outing at a more convenient date.

Starting to see why Harry really doesn't like getting on a plane now though

::mental note add motion sickness tablets to cunning kidnap caper/plan/thingy...::