I think that I finally come up with a solution for this issue.   The solution allows me to assign values to choices in a drop-down list from a lookup rule.   Below are the steps that I used.  I am using students (student number and student name) as my example.

1.  Write a view/stored procedure that returns the value and the text that you want all in one field with some sort of unique separator between the two parts of the field. In this example I am using the pipe character (|) as the separator.
   (e.g SELECT '0001' + '|' + Jane Doe' AS [StuNoAndDesc])
2. Create your lookup rule as normal, populating your drop-down list with the concatenated field.
   (e.g. Fill Student with result set column StuNoAndDesc)
3. Use JavaScript to loop through all of the options returned by the lookup.  Split the field based on your delimiter character and assign the id value (StudentNumber) to the value option of the drop-down list and the description value (StudentName) to the text option of the drop-down list.

Here is a sample of the JavaScript: 

$(document).ready(function () {
  
	//Disable the Student Selector drop-down.
  	//Re-enable it later in the code after it has finished loading
  	$('.Field_StudentSelector select').prop('disabled',true);
  
    //This will attach an event that will only run once.
    //It will be run once ajax has completed
    //,which is when the lookup on a field has completed.
    $(document).one('ajaxStop',function(){ 
          //Loop through all options in the drop-down list.
          $('.Field_StudentSelector option').each(function(){
              //Split the text option on the pipe and take the first part
              //assign this as the option value.
              $(this).val($(this).text().split('|')[0]);
              //Split the text option on the pipe and take the second part
              //assign this as thetext value.
              $(this).text($(this).text().split('|')[1]);
          });
          //Re-enable the drop down now that it has completed loading
          //and had the option values and text set.
          $('.Field_StudentSelector select').prop('disabled',false);
      });
  
  //When a value (student) is selected get the
  //selected value from the drop-down and put into
  //the StudentNumber field.
     $('.Field_StudentSelector select').change(function () {
    	$('[attr=StudentNumber] input').val($(this).val());
  });
  
});

Here is sample of the outputted Html elements:

<ul>
  <li attr="teststudent" attrtype="select" name="q28" id="q28" class="Field_StudentSelector form-q label-left form-focused">
	<label class="cf-label" for="Field28">Student</label>
  </li>
</ul>
<div class="cf-field">
  <select id="Field28" name="Field28" aria-label="Field28" class="form-item-field cf-medium user-success" vo="e">
	<option value="0001">Jane Doe</option>
	<option value="0002">John Smith</option>
	<option value="0003">Zhang San</option>
	<option value="0004">Guiseppe Verdi</option>
	<option value="0006">Hans Schmidt</option>
	<option value="0006">Rajwinder Kaur</option>
  </select>
</div>

Thanks to Jonathan Rowe for pointing me in the right direction and for the tip about using ajaxstop as a method of ensuring that the drop-down has finished loading before processing the additional Javascript.  (more info in this post).