Copying files to the clipboard using applescript
This isn't quite perfect for what you want, but I think it might get you closer. Quicksilver has a command line tool. When installed, you can pipe files from the command line to Quicksilver app, and from there send them as attachments in Mail.

To install Quicksilver command line tool, activate Quicksilver. Then navigate to Quicksilver > Preferences to open preferences window. Click on Preferences on top right of window. On the left, click on Command Line Tool and install.

In Terminal, you'll be able to type 

qs path/to/file1 path/to/file2 path/to/file3 path/to/file4


Pressing enter will send those files to Quicksilver's first pane. Press tab to move to the action pane and choose the Mail/New Email With Attachment action. Press enter, and a new email with your selected files attached should appear.

Note that you will also have to install the Mail and Command Line plugins in Quicksilver (Quicksilver > Preferences > Plug Ins).