I’m not too handy with building anything more than legos, but I was wondering…
How difficult would it be to turn an Etch-A-Sketch into a controller?
I was thinking of how fun it would be if I replaced the stock knobs with moog ones when I fantasized about an Etch-A-Sketch with X and Y outputs.
Is it possible to make one and still keep the screen intact?
You can put circuitry into anything. I’ve never seen the inside of an Etch-A-Sketch. I would think things would get really messy really fast. There was a fine black powder in them as I recall. Not sure if you could have a controller built into a fully functional one. 
Now THAT’S something to think about…a fully functional Lego controller! I’ll bet with the new computer-controlled kits they now have, you could build an MP-201 out of Legos! 