Can non-involved player use abilities during battles?

i.e During a battle between Player A & B : Can Player C use Metalworking to add a bonus to a unit of player B?

I'm guessing not but thought I check as Metalworking lacks a definitive "your unit" or "your hand" or "your attack"

If you read the Faq, it specifically talks about this card and the errata states playing a combat unit from "your" hand.