If someone did happen to know what is behind there, even if they said, "yes there's room", or "no, there's not much room", you'd still have to take it off to see if your particular switch would fit in whatever room there is
I apologize for sounding curt, I should have written out the whole thought the first time.
4W2FJC, this video will answer your question and also give you instructions on how to remove the nightmare that is the driver's side dash. At 2:17 into the video, it clearly shows what's behind the area you pointed out, but I don't know how much room there is for a switch. It's mostly support plastic and no components at least. Since it looks like such a PITA to remove, you might want to explore more easily accessible areas for your project.
@4W2FJC Don’t forget you’ve got all the pop out switch covers. Depending on the options your FJ has you might have one or two free to the left of the steering wheel. And while harder to reach and see, there’s the bank of switch covers below the radio. It all depends on what you’re going to use the switch for, and how easy/frequently you need to access it or how visible you need it.