Hm, I might follow that idea (open in a new terminal window or split screen). I'd somehow have to get the original message as a quote for the new message.
You can always open the mailbox first and grab the message you want to reply to:
mutt -Rf <mailbox name>
... will open the given mailbox in read-only mode (so as not to update read flags and such).
In practice, I generally run mutt within screen, with multiple frequently-accessed mailboxes open. I switch mailboxes by moving between screen buffers.
