This was posted in a beaglebone tweet. It looks like there's been some improvement in the compatibility between the beaglebone black and the beaglebone AI, thanks to Deepak Khatri in the 2020 google summer of code (GSOC).
Introduction:
https://www.youtube.com/watch?v=jP9fwOxp4Bc
Follow-up:
https://www.youtube.com/watch?v=jxqSYCKa3K8
The stated goals look good, it's what I'd like to have.
If I am following the explanation in the video correctly, he has individual compatibility, I2C, SPI, ADC, etc, in each device tree overlay, but not all in the same device tree overlay. The objectives stated sound good. I guess I'll have to finally learn to manipulate the device tree.