I thought we could do hidden objects right in GM mode? Can't we just 'de-activate' objects and it makes them invisible to the players? Then right click and 'activate' them to make them show up? I do this in my campaign and it works fine. I know you can do it with creatures. I'd image you can do it with levers, chests, traps, you name it.

GMAddon1: Adds almost every single monster and NPC from the main campaign into GM mode, plus a few large custom maps to use in GM mode.