I was under the impression that in order to heal w/ Brass Serpant you had to have a hero to play it on, is this not the case?
That is true, you must have a character to activate any enhancement. Healing cards are a special exception in that the Hero does not have to be in play, it can also be in the discard pile provided it was discarded on the same turn you heal it.
Here's a clarification that's being added to the REG:
A healing enhancement played in this way must be played on a character of matching brigade in one of two ways: (1) on a character in the holder's territory or (2) on a character in the holder's discard pile (if the character was discarded in the same turn). Both result in that character being healed. Healing cards played on a character in a discard pile must target the same character.
Related Q, same scenario: Can I use I Am Healing to heal the heroes, or since it says 'in play' would it not be able to?
I think the wording of I Am Healing would limit it's targets.