Here is a Unix script that I use to create an email in Mail containing a number of attachments that I specify via the command line. If you save the script as, say, attachments, then you create the email by entering something like attachments foo.jpg bar.txt. Remember to chmod +x the script and move it into someplace in your path, like /usr/local/bin.
#!/bin/sh
FILE_LIST=""
while [ "$*" != "" ]; do
FILE_LIST="$FILE_LIST , POSIX file \"$1\""
shift
done
osascript \
-e " set file_list to { $FILE_LIST } " \
-e " tell application \"Mail\" " \
-e " open file_list " \
-e " activate " \
-e " end tell "
Note that the obvious way to do this with a droplet and on open fails to work because using the open(1) command causes the on open to receive the first file passed in, rather than the list of files. I imagine this is a bug.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041007213921761