It *probably* is not really what you're looking for, but Mutt can be configured to use any tool that can understand (or be made to understand via a wrapper) good ol' sendmail's parameters, or which can expose a SMTP submission interface on a TCP port; and you could set up key bindings to choose one or the other via Mutt's macro support. And I'm quite sure it's GPL2 or maybe GPL2+, which should meet your FOSS criteria.

Let me know if you get anything working! Sounds like an interesting idea