You should be able to echo values, so there must be an issue with how you are adding it. Where do you want the value to come from? Is the readerid in the recordset you are using to send the emails and get the TO address?
Yes - the readerid is in a recordset being used in the page (sending the mail) as is the TO address
The content of the html email is stored in a mysql table, in a text column, so for example it the content of the text column might look like this...
<p>some content here</p>
Hello <php echo therecordset['Firstname']; ?>
<p>some more content here</p>
This column is then referenced to in the page (the page sending the mail) as the content.
So when I open the dialogue box and choose the content I choose the recordset column as defined.