It is "Bill Gates" that told me it was possible placing observation rooms on the floor, so credits to him :) I tested a few things and found a trick to do it in Portal 2 editor.
It could be used for a lot of purposes, as decorative ones or disorienting ones.

Usually, observation rooms can only be placed on walls. But a little trick let you place it on the floor. While in the editor, you need to move your puzzle (by right-clicking, left-clicking and dragging it) until the floor where you want to place the observation room is exactly behind the observation room icon in the left menu, when it is opened. Then you just left-click on the icon and left-click on the floor and this is it !

Look at the video to make things clearer :

There are a few things that are pretty important when trying to place observation rooms on the floor. You should always make sure there isn't a wall next to the floor surface on which you want to place the observation room because if that's the case the observation room will place itself directly on the wall.
Also remember that you only have to do the manipulation once, because you can copy with ctrl-left-click the floor observation rooms.

Look at the video to master the floor observation rooms placement :

With this trick you can add a new dimension to your maps, disorienting the player or adding intersting spotlights. Here is an example of what you can do with this : https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=397482930 Enjoy !