You could do it crudely with a trigger_camera that points to a rooms outside your normal map, textured black. Then you could use env_renders and/or func_door_toggles to make "items" in your inventory appear. (this might work for things like keycards and such)
If you need to be able to select items, use a trigger_teleport instead, temporarily sending the player to a place he can physicallly pickup/activate/use things in the inventory.