iText PDF Forms: How do you change the tooltip for a field?

I'm using the iText library which is baked into ColdFusion to rename PDF form fields. I can rename the field name easy enough, however the resulting PDF shows a "tooltip" when you hover your mouse over the field which does not match the new name of the field.

I did some research and it looks like the tooltip is stored in the "/TU" key in the field dictionary and it can be set with the setUserName() method. However, when I examine the various objects in my code, I don't see any way to gain access to that method for a particular PDF field.

How can I set this "/TU" key using the iText library?

Here's my code so far:

 function renameFields( pathToFile ) {

    // initialize the PDF file we will be working on
    local.pdfService = new pdf();
    local.pdfService.setSource( arguments.pathToFile );
    local.pdfFile = local.pdfService.read();

    // initialize the iText library objects
    local.pdfReader = createObject("java", "com.lowagie.text.pdf.PdfReader").init( toBinary( local.pdfFile ) );
    local.outputStream = createObject("java", "java.io.ByteArrayOutputStream").init();
    local.pdfStamper = createObject("java", "com.lowagie.text.pdf.PdfStamper").init( local.pdfReader, local.outputStream);

    // get an instance of the acro fields object
    local.acroFields = local.pdfStamper.getAcroFields();

    // Get All of the Fields of the PDF
    local.allFields = local.acroFields.getFields();

    // convert the collection of fields into an array for easy iteration
    local.fieldArray = listToArray( structKeyList( local.acroFields.getFields() ) );

    // loop through all fields and rename them
    for ( var a=1; a < arrayLen( local.fieldArray ); a++ ) {

         // do the actual renaming
         local.acroFields.renameField( local.fieldArray[a], "field_#a#" );

         // update the field tooltip ???

    }

    // finish up and return the pdf file object
    local.pdfStamper.setFormFlattening( false );
    local.pdfStamper.close();
    local.pdfReader.close();
    local.myPdf = local.outputStream.toByteArray();

    return local.myPdf;

}