You would have to add your option to the SHOWTHREAD template, by editing the template or by doing a replacement in a plugin. Then you'd need a plugin using hook location threadmanage_action_switch to handle when it's selected (it's probably best to look at postings.php and search for that hook name to see how to handle it).
What do you mean by a replacement in the plugin? I was hoping I wouldn't have to edit any templates. Blergh.