I ran some test this morning and that's what i found.
In the first picture you can see it seems fine. I put attract mode in window instead fullscreen and give it a 4:3 aspect ratio.
In the second photo i loaded my hyperspin.nut in your hyperspinv3 image for raspberry but it is in a 16:9 tv.
In the third photo i just forced in config.txt file a resolution of 1280x960 60Hz 4:3 in my tv
that's what i change in config.txt:
hdmi_group=2
hdmi_mode=32
but it works like that just if in the display layout option i leave in 16:9. If i set in 4:3 appear black bar on top.
In raspberry it could depend from your hardware resolution, from monitor setup and also from config.txt.
Every setup could be a little bit different you have just to play a little bit with the option.
And like i said early i'm not a developer, i work in the kitchen but i have a great passion for tech.
The modification that i maked here and also the tip i gave you come from many years of trials end errors with raspberry pi and also pc!!!
