I made the mistake of using a poison dagger while it was unequipped. It does damage the creature on the square and counts as your dagger use for that round (IOW you can't then equip it and poison until retreat, just as if you'd used it proper) However it does not give the poisoned message, weaken the creature overall, or use a charge from the dagger.
So a minor bug. This may be along the same vein of people being able to use other's charge items, thought I'd add it anyway.