Permit link in caption

It appears that captions are stripped of all html tags, with the result that it's impossible to provide a link (to a high-resolution version of the image, in my case). For a very simple example, if you put link into a custom caption, it is rendered as the word "link", with no link tag. In my case, I used a pre_render function to add the link html to an image field as follows:

      foreach($view->result as $key => $row) {
        if(isset($row->field_field_highresimage[0])) {
          $url = render(file_create_url($row->field_field_highresimage[0]['raw']['uri']));
          $markup = '<a href="'.$url.'" target="_blank">download high-resolution image</a>';
          // add the markup to the colorbox_custom_caption for the regular image field (which is displayed in a colorbox)
          $view->result[$key]->field_field_image[0]['rendered']['#display_settings']['colorbox_caption_custom'] = $markup;
        }
      }

but it is rendered just as the text "download high-resolution image", no link.

Presumably somewhere downstream of this, a strip_tags() call is made, but I have no idea where.