Thanks for the replies. I ended up using the mail.box technique. In case anyone else needs it, here's the code:
Create the body field.
var body:NotesRichTextItem = doc.createRichTextItem("body");
body.addNewLine();
body.appendText("Four score and seven years ");
body.addNewLine(2);
body.appendText("ago our fathers brought forth on this continent, );
body.addNewLine(2);
body.appendText("a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal..");
body.addNewLine(2);
var subject = "The Gettysburg Address"
var emailAddr = "fan@there.com"
sendEmailViaMailBox(emailAddr, subject, body);
function sendEmailViaMailBox(emailAddr, subj, rtBodyFld){
var mailboxdb:NotesDatabase = sessionAsSigner.getDatabase("<server name>", "mail.box");
var emaildoc:NotesDocument = mailboxdb.createDocument();
emaildoc.replaceItemValue("form", "Memo");
emaildoc.replaceItemValue("sendTo", emailAddr);
emaildoc.replaceItemValue("Recipients", emailAddr);
emaildoc.replaceItemValue("subject", subj);
//emaildoc.replaceItemValue("body", rtBodyFld);
emaildoc.copyItem(rtBodyFld, "body");
emaildoc.replaceItemValue("SMTPOriginator", "xyz@abc.com");
emaildoc.replaceItemValue("From","\"xyz@abc.com\" <xyz@abc.com>");
emaildoc.replaceItemValue("Principal","\"xyz@abc.com\" <xyz@abc.com>");
emaildoc.save();
}