Ok so I worked out how to do this for version 2.6.4.
there are three files you need to edit:
- modules/form/modify_field.php
- modules/form/save_field.php
- modules/form/view.php
modify_field.phpInsert this at Line 171:
<?php if($type == 'textarea') { ?>
<tr>
<td><?php echo 'Read Only'; ?>:</td>
<td><input type="radio" name="readonly" id="readonly_true" value='readonly="readonly"' <?php if($form['extra'] == 'readonly="readonly"') { echo ' checked'; } ?> />
<a href="#" onclick="javascript: document.getElementById('readonly_true').checked = true;"> <?php echo $TEXT['YES'] ?> </a>
<input type="radio" name="readonly" id="readonly_false" value="" <?php if($form['extra'] == '') { echo ' checked'; } ?> />
<a href="#" onclick="javascript: document.getElementById('readonly_false').checked = true;"> <?php echo $TEXT['NO']; ?> </a> </td>
</tr>
<?php } ?>
save_field.phpInsert this at line 33:
$readonly = $_POST['readonly'];
Change line 79 from:
$database->query("UPDATE ".TABLE_PREFIX."mod_form_fields SET value = '$value', extra = '' WHERE field_id = '$field_id'");
to:
$database->query("UPDATE ".TABLE_PREFIX."mod_form_fields SET value = '$value', extra = '$readonly' WHERE field_id = '$field_id'");
view.phpLine 173 from:
$values[] = '<textarea name="field'.$field_id.'" id="field'.$field_id.'" class="textarea">'.$value.'</textarea>';
To:
$values[] = '<textarea name="field'.$field_id.'" id="field'.$field_id.'" class="textarea" '.$field['extra'].'>'.$value.'</textarea>';
Thats how i done it. I'm not sure if its the most efficient way but IT WORKS

Cheers
-Ben