I want to use mh-e to write mail, however message.el does not permit 
to use other MUA. So I wrote a patch for message.el.

  This patch expands a variable `message-buffer-constructor-alist'.
This variable is an alist which key part is buffer name and value part
is functions to build message buffer. Default key is `t'.

  This patch does not have implementation for other MUAs. I wrote
buffer-constructor for mh-e and inserted to gnus-mh.el. In addition,
I modified gnus.el for auto-loading.

  Maybe you can write other buffer-constructor if you want to use
other user-interface to write message.