Justin,
Custom Text message codes should be processed before other codes, so you could set up a footer in one of those and include a Remove Link code in it. Then you would simply use the one code at the end of your messages. If there are any problems with that, you'd have to use two codes. While it may be possible to set up a hack to automatically add codes or text, it can't be recommended or supported. To put you on the right track, you might add to the message body before the call to the processmsg() function in admin.php.
As of v1.88, that occurs on line 2411:
list($xsubj,$xmsg,$xhtmsg)=processmsg($userid,$subj,$msg,$htmsg,'0',$typ,$list);
So before that you might put:
$msg.="!custom";
$htmsg.="!custom";
This will add the !custom code to both the text and HTML message bodies. The code will then be processed by the processmsg() function call on the next line.
\n's should be interpreted properly if between double quotes within PHP script. If using single quotes they will not be interpreted. When editing a message code within ListMailPRO simply hit Enter to add a line break.
This modification may have unforeseen consequences with certain messages or notifications and cannot be guaranteed to work as expected.
Regards,
Dean