So I had a situation where I just HAD to control the message form which isn't supported for client script yet.

In a nutshell:
1)  I needed to insert a fax number that wasn't the entities, but rather a child custom record value.
2)  Set the subject (and disable the field)
3)  Change the entity status
4)  Verify required data was present
5)  And the really tough one... choose an attachment template from the list.

I wanted to share my solution in case it helps anyone and also ask the pros if there is a better way to do what I did.

I still have a couple of problems to sort out, there is a step or two that I'm missing with setting the drop down list, when I set it, I'm getting an unexpected error upon submitting...  more research needed.