This is not possible.
Mail's AppleScript API also does not allow for proper RTF mail. The following fails to create a proper mail body with links and everything, something clearly there in /Developer/Applications/Utilities/Clipboard Viewer (part of Xcode 4):
tell application "Mail"
set theRTF to the clipboard rule «class rtf »
set msg to make new outgoing message with properties {subject:"Test", content:theRTF}
make new recipient at end of to recipients of msg with properties {address:"danielbeck@example.org"}
send msg
end tell
You can try to use GUI scripting to copy&paste rich text using the clip board and the actual Copy/Paste menu items.