A raíz de una consulta en el grupo Catalis, acá va una receta para lograr que un subcampo se pueda editar de manera análoga al subcampo $4 de los campos 700 (relator code).

cd htdocs/catalis_pack/catalis
$ find . | xargs grep -il relator | grep -v 'loc/marc'

./xml/marc21.xml
./xml/relator.xml

./html/editCodes.htm

./js/create-field-subfield.js
./js/validate.js
./js/aux-windows.js
./js/punctuation.js
./js/catalis.js


XML
    frequency.xml

HTML
    ------------------------------------
    editCodes.htm
    ------------------------------------

		case "relator" :
			// TO-DO: pensar de qué manera se puede acotar la larga lista, en función del tipo
			// de documento (video, musica, libro, etc.)
			xmlCodes = callingWindowObject.xmlRelatorCodes;
			Codes = xmlCodes.selectNodes("/" + "/relator");
			title = "Relator codes";
			break;
			
		case "frequency" :
			xmlCodes = callingWindowObject.xmlFrequencyCodes;
			Codes = xmlCodes.selectNodes("/" + "/frequency");
			title = "Frequency codes";
			break;

JavaScript

    ------------------------------------
    create-field-subfield.js
    ------------------------------------

	// Subcampos cuyos valores se toman de tablas
	if ( "4" == code ) {   // Relator code
		newSubfieldBox.readOnly = true;  // ATENCION: esta propiedad produce un bug al subir/bajar un subcampo $4 vacío
		newSubfieldBox.onclick = function() {
			editCodedData("relator");
		}
		newSubfieldBox.style.fontFamily = "lucida console";
		newSubfieldBox.style.fontSize = "14px";
		newSubfieldBox.style.lineHeight = "17px";
	}
	
	else if ( tag_code.search(/86[34]w/) != -1 ) {  // Frequency code.  Expresión alternativa: /863w|864w/ 
		newSubfieldBox.readOnly = true;
		newSubfieldBox.onclick = function() {
			editCodedData("frequency");
		}
		newSubfieldBox.style.fontFamily = "lucida console";
		newSubfieldBox.style.fontSize = "14px";
		newSubfieldBox.style.lineHeight = "17px";
	}


    ------------------------------------
    aux-windows.js
    ------------------------------------
    
    function editCodedData(dataElement)
    {
    	if ( dataElement.search(/relator|frequency|f041|f044/) != -1 ) {
    		var srcObject = event.srcElement;
    		var activeCode = srcObject.value;  // TO-DO: evt for mozilla
    		var codeType = "single";
    		var dialogLeft = event.clientX - 70;
    		var dialogTop = event.clientY - 130;
    		var URL = URL_EDIT_CODES;
    	} else if (
    	
    	...
    	
    	if ( dataElement.search(/relator|frequency|f041|f044/) != -1 ) {
    		srcObject.value = newCode.value;
    		//displayPermanentTitle(srcObject,newCode.description.substr(6),40,0);
    	} else {
    	...
    }
    
    
    ------------------------------------
    catalis.js
    ------------------------------------
    
    URL_RELATOR_CODES = HTDOCS + "xml/relator.xml";
    URL_FREQUENCY_CODES = HTDOCS + "xml/frequency.xml";
    
    xmlRelatorCodes = importXML(URL_RELATOR_CODES);
    xmlFrequencyCodes = importXML(URL_FREQUENCY_CODES);