Ok, so Trolls and Dwarves have been around for quite some time now, so you would think someone would have developed an ink, probably involving some sort of biogel, that emanates heat, just enough for troll or dwarf eyes to pick up. This ink might be widely provided as a novelty item by some of the more meta-friendly corps, and might see a lot of use for communication between troll gangers and such. It would be an ink that only Dwarves, trolls, and people with expensive, specific cybereyes could see.

actually it seems like it would be easy to do the opposite as well and have shirts and tats that don't conduct heat well. They would still show up very well against a person's heat.

Maybe a human that wears a shirt like this that says "I love my woodland friends" in print but in low conduct it says, "If you can read this you should crawl back under your bridge". What would be even better is give it to a human and not tell him.