Right now items can be tagged [M]elt [D]ump [F]orbidden and [H]idden. If [T]rade were to be added; what other flags might be added at the same time in a batch to encourage Toady to make the multiple changes at once?
oak to command an item be dropped in a Pond zone (cleaning off contaminants)
ncinerate to command an item be dropped into the nearest magma or furnace to be destroyed, without material recovery (zombie body parts)
[A]mmunition to load an item into the nearest catapult for firing ("fetchez la vache")
Bit flags come in batches of 8, so if Toady is only using 4, then he has 4 that aren't being used right now, anyway, unless he did some sort of other hidden strangeness for data compression we don't know about.
However, if we have farming fertilizers to worry about, a composting option for organic materials like used clothing or dead goblins would be nice.
Also, "clean this" and "burn that" options would be good ideas, although I believe you forgot that brackets are used for markup text.
In addition to those, we could also have a special marker for allowing something to be claimed as internal fortress property, or, inversely, a ban on allowing internal fortress members to own that item as property. For example, marking goblets as not for sale to outsiders, but OK for dwarves to own their own goblets.