I would like to agree with this idea in full. Recently, I had to flag a post and send a PM, and was also surprised that I was not able to customize the message.
First, I have never seen the message myself, so I have no idea what exactly was said to the user! Second, my guess is that the message is a generic (this message violates the site rules and guidelines, please edit) sort of thing, and I would have wanted to customize it. In my case I would have wanted to explain that the use of http://lmgtfy.com is not allowed on our site, and why it was rude and incosiderate to do so. Instead, the user edits the post asking who flagged it and why (resulting in me flagging and hiding it again!) and having to manually send a PM explaining the situation.