Are there any special tools, materials or techniques required for building a staircase door?
From this picture it hardly looks like the stairs "cuts the room in half". In fact it looks like the stairs hug all the way to one side of the room. 

I would strongly discourage you from proceeding with the idea that you propose. If the horizontal trap door were closed in the event of a fire it may very well be just that...a trap the prevents you from escaping in a timely manner, if at all.