&eFormOnBeforeMailSent I think is the event that you would use here.
ThegetTemplateVarOutput takes the following parameters:
$modx -> getTemplateVarOutput( TV name, document ID, published state );
I am still not too sure how to go about this, I do see that FDM can save unpub dates, this is included in the db class and I was able to hard code a value. But I did not recognize any function to address this in the FDM class (I don’t really understand classes so it could be there). As I said I don’t understand classes, but in theory you should be able to add this in with a few lines, there is code to set the pub date so I would try to copy and modify it to accommodate the unpub date value.
NewsPublisher seems to record unpublish dates, you should be able to edit it on the fly by modifying this line to do the math:
$unpub_date = $_POST['unpub_date'];
Maybe something like $unpub_date = ($_POST[’eventDate]+86400); or something like that.