What happens?

You're disarmed, and you drop the weapon. But you can pick it up again, and I believe it will still be tagged as a quest item.

Even though quest items can't normally be dropped?

Yep, I'm almost 100% sure that's what happens. Unless it's dropped into a dark part of the dungeon.. Then you're kind of screwed.

You've got me curious about this, though I bet it wouldn't work. Probably some extra code specifically for that.

I'm curious to know what quest item could you possibly be holding while facing a draugr that uses disarm?

Dawnbreaker?

Anything that can be smithed.

I was too. It would be nice way to get rid of a quest item you didn't want, but had no other way to drop. Or, it would be a nice way to gimp a miserable shout. "Ha-ha. Not today buddy!"

I've got a character suitable for the job. Might venture into the Ratway for Dravin's Bow, then give a Dragr a visit. Lest someone has tried it.

Nettlebane, Amren's Family Sword, Wuuthrad (before going to that place), or Red Eagle's Sword come to mind easily, before the quests are completed.

I am curious, too, about the question. You can not drop quest tagged items in the game. I have tried. I may have to try this out