Okay. I looked into this, and I learned something new. For starters, discard is actually different to smashing a huge pumpkin. Smashing a huge pumpkin indeed does appear to be an alternate form of inv_use.php (which is why I proposed an altuse command). Discard, on the other hand, is a completely separate inventory action altogether.
Secondly, I learned you can actually discard pretty much anything in your inventory that isn't marked as "Cannot be discarded" (funny about that). I haven't tried discarding an IotM (any takers?) but most (if not all) appear to be marked "Cannot be discarded" anyway.
I haven't attempted to discard any of the quest items that are not marked as "Cannot be discarded", of which there seems to be a few in the nemesis quests, but I currently have none to test this with.
I can also confirm that you can only discard a single item at a time (such as 1 useless powder at a time).
So the million meat question, do we open Pandora's Box and create a "discard" command that can be used for all discardable items?