How to set readonly or locked a select webform field?
Not sure how much this is a helpful answer, but was looking for a recipe to do this and found this example using a simple little hook_form_alter (which happens to be for civicrm components in a webform but think the principle is what counts)

$form['submitted']['civicrm_2_contact_1_fieldset_fieldset']['civicrm_2_contact_1_relationship_relationship_type_id']['#disabled'] = TRUE;