The problem is that although you can put an RTF link on the clipboard, an application needs to know what it is in order to use it as a link. You can paste the link into various applications, but unfortunately AppleScript isn't one that understands it, so you can't just put the link text into one of the properties of your message. GUI scripting the New Message window can be a pain, as is most GUI scripting, so maybe using signatures will do the trick.
Once you get the link text onto the clipboard (using
this script from Ahsan's post above, for example), go into Mail's preferences, create a signature, and paste the link into it. Then, just modify your own script to include it, for example:
<pre style="
font-family: Monaco, 'Courier New', Courier, monospace;
font-size: 10px;
margin: 0px;
padding: 5px;
border: 1px solid #000000;
width: 720px;
color: #000000;
background-color: #FFDDFF;
overflow: auto;"
title="this text can be pasted into the Script Editor">
tell application "Mail"
set theSubject to "some subject"
set theContent to return & "test" & return & "testing" & return
set theSignature to signature "Signature #1"
tell (make new outgoing message)
set visible to true
set subject to theSubject
set content to theContent
set message signature to theSignature
end tell
activate
end tell
</pre>
I used a default signature name "Signature #1" in the above example - change it to one of your signatures (obviously). There is also an example Mail script in
/Library/Scripts/Mail Scripts/Create New Message.scpt you can use to get the various properties into your outgoing message.