If you put the send mail button inside the portal row, the mouse click on the button will also make that portal row the "current" row. Thus the Send Mail Dialog can refer directly to the notes field in this portal and the note of the clicked portal row should then appear in the body. Your To box can still refer to the parent record for the email address.
Worked like a charm... I had it set up that way originally but my mistake... I referenced the wrong table for the notes in set-up dialog box.