Users of notmuch may be interested in Arisawa-san's port of the
wanderlust template mechanism to message mode.  This allows for example
to emulate "gnus posting styles" (at least as far as I understand them)
and to send form letters.

I have made a public git repo containing Arisawa-san's version and the
beginnings of some enhancements. You can find it at 

     http://pivot.cs.unb.ca/git/?p=message-templ.git;a=summary

David