Feruchemical tattoo...

I've been seeing Mistborn-themed tattoos featuring Allomancy and Feruchemy symbols. I just realized that, in-world, it might theoretically be possible to create a Feruchemically active tattoo—by infusing tattoo ink with the appropriate metal in a high enough concentration, it could function as a metalmind.

Since Allomancers are naturally immune to poisoning from the metal they burn, it's likely that Feruchemists would have a similar resistance to metal poisoning so they should be be fine (Wax and Wayne do put their metal inside their body without any negative impact l)