several teachers asked me for this feature so i patched the "assignment/submissions.php" code
to include an ability to add notes or send messages to selected students on the list.

all changes are in mod/assignment/lib.php

since i can not nest html forms (it is not in the html standard) i had to choose between
the form that is generated (wrapping the flexi table) by the $quickgrade option and
this new "send-message" form.

btw,
i tried to have a javascript code change the ACTION in the form and trick it to have two different
submit buttons. it worked to some degree but my lack of knowledge in JS held me back from fully
implementing it successfully. so i revert to this simple solution of one form.

i am attaching a diff file against moodle version 1.9.2 csv code.
in an acrhive that includes my version (lib.php) and the cvs version i got from
http://cvs.moodle.org/moodle/mod/assignment/lib.php?revision=1.277.2.42&view=markup&pathrev=MOODLE_192

patch-moodle/lib.php
patch-moodle/lib.php.cvs
patch-moodle/lib.php.diff