More Fridge Logic than JBM, but since it's not related to the actual plot I'll put it here - what happens if you put toys representing fictional things into the cupboard? E.g., a toy lightsaber, or a Gorgonite?
Lightsabers aren't fictional! They're just ancient technology from a long, long time ago. And maybe Gorgonites are from the future?
Gorgonites are obviously from the present, or very close to it. They also happen to be toy-sized, so you'd just end up with a life-sized Gorgonite.
It was shown to bring Darth Vader to life in the movie, so it probably would make fictional things real.
Based on how it works in the books, (not the movie, apparently) it would probably bring the prop or the actor who was portraying the character at the time.
Yeah. If you put a Darth Vader figure in the cupboard, David Prowse or perhaps Bob Anderson would take off the helmet and ask "Where's George?"
If you got the exact right Darth Vader figure and good luck finding that. For the vast majority of Vader figures you would just give some guy the BEST COMIC-CON EVER!