Hmm, well he could post the results in ungraded quizzes for now (as a series of description 'question' types), since quizzes don't allow guests to enter them.

Since this means there is a setting, one could locate it in quiz and perhaps make a type of resource that also doesn't allow guest entry.

I had the same interest in opening the course to everyone, but hiding some resources.
Here is the solution:

1. in /mod/resource/type/file/resource.class.php in the function display() before the comment "Form the parse string" include the following new lines:

    if ($resource->blockguests == 1) {
        require_login($course->id);
        if (isguest($USER->id)) {
            error(get_string('noguestaccess', 'resource').".");
        }
    }

2. with phpMyAdmin or some other tool, extend the moodle data structure in mdl_resource with a new field:
name: blockguests
type: tinyint(4)
Default: 0


3. in /mod/resource/type/file/file.html behind the line "<tr><td colspan="2"><hr /></td></tr>" insert the following lines:

<tr>
    <td>&nbsp;</td>
    <td>
      <input type="checkbox" name="blockguests" value=1 <?php echo ($form->blockguests == "1") ? "checked" : "" ?> >
      <?php print_string("blockguests", "resource") ?>
    </td>
</tr>

4. in /mod/lang/en/resource.php insert the lines (alphabetically):

$string['blockguests'] = 'Block access by guests';
$string['noguestaccess'] = 'Sorry, no guest access to this resource';

5. in /mod/resource/lib.php within the function function update_instance($resource) behind the line "$resource->timemodified = time();" insert the following lines:

    if (isset($resource->blockguests)) {
        $resource->blockguests = 1;
    } else {
        $resource->blockguests = 0;
    }

6. Testing:
When adding a resource, you should now see the additional option "Block access by guests". By selecting this option and saving the changes and then editing the resource again, you should see your selection (i.e.: the setting has been saved / recalled successfully).  Now, browsing the course as guest, you should have access to all resources but the one marked as "block access by guests".

Good luck,
Klaus