I've just uploaded a new version of this plugin. It fixes bugs with certain items not showing up and adds a couple of new features.
Note that the hotkeys have changed... I've made the flag toggling use Ctrl-hotkeys, so that I can use more sensible Shift-hotkeys for the dump and forbid actions (the OP has been updated with new screenshots of these).
There is "Trade" flag indicating items marked for hauling to the depot. This does not include items at the depot; those are considered "in a building" and not shown in this screen. There's now a "Mark for Trade" option that lets you set one or more items for hauling to the depot. Due to complications, you can only mark them, you can't unmark them once they are marked. You would have to use the Trade Depot hauling screen if you want to cancel that job. This option won't mark artifacts, items in inventory, owned items, items in a job or items that match current export restrictions. If you mark an item in a container, the container will be marked for trade, but only if every item in it is valid for trade according to those conditions. When a container is marked for trade, all items in that container will show the Trade flag.
The "valid for trade" check in the plugin may be a bit too restrictive when it comes to containers... I've noticed that the base game lets you mark bins for trade even when they contained items still owned by a dwarf. I wanted to stay cautious about that for now so the plugin won't mark a container if it contains any owned items. The next version will probably allow that, once I confirm it really is ok to do so.
Let me know if you notice any bugs.