Need to be able to update a Custom Data but nothing else
You might want to consider using webform_civicrm module. I don't recall how up to date it is with drupal 6 but it should allow you to give your commentators a form which is restricted to just that role, and which has an autocomplete to find the contact and the desired custom field to enter their comment in.

it might even be better if you use the form to add the 'comment' as an Activity if there is the possibility of many 'commentators' adding comments.