Thanks, that source of information worked out for me. For the benefit of others, here is the Adobe Javascript Scripting Reference document he identifed:
http://partners.adobe.com/public/developer/en/acrobat/sdk/AcroJS.pdf
... and then the help I was looking for is the desciption of SignatureInfo object. In the Livecycle javascript in my form, I can now go to the desired username attribute as follows
var digSig = event.target.getField ("MyForm[0]...MyPathstuff [0]...Signature [0]");
digSig.signatureValidate ();
If successful...
var sigInfo= digSig.signatureInfo (); // Gets SignatureInfo object
var sSigner = sigInfo.name;