To record a movie with an added background I suggest recording all the movie frames with transparency and then combining those frames with the background image to make a movie.  Here is an example of how to do this.

open 8of8

movie record dir ~/Desktop/movie transp true format png ; turn y 3 120 ; wait 120 ; movie stop

       # Let's put a cryoEM map in the background, open it and zoom in by hand

open 16850 from emdb

save ~/Desktop/movie/background.png

Now to combine the transparent image frames and the background into a movie I'll use the ffmpeg movie encoder that comes with ChimeraX in a shell window.

$ cd ~/Desktop/movie