Perma-labelers, for Security and pranking!

[warcrimes]

Sounds mad abusable if it functions the way the labeller does- You can't hide it, you can't change it, it's either going to suck all the mystery out of a stealther's round, or cover people in slurs.

How about a compromise: tattoo gun.
aim at a body part, write stuff in, somewhat longish loading bar (so your target is willing or restrained) and then it's visible as long as that part is uncovered.
permanent, conspicuous, but manageable.