Jacek Rudowski

Update: Yes, a dtbo will work to reduce the bus from 8-bits to 1-bit and make the board bootable.
 
This overlay does the trick for me:
sun8i-r40-bananapi-m2-ultra-1bit-emmc.dtssun8i-r40-bananapi-m2-ultra-1bit-emmc.dtbo
 
To test this for yourself: (You will need a working Linux machine to mount the SD card and make these changes.)
Burn a copy of the current rolling release image onto a microSD card. Copy the dtbo file into the /boot/overlay-user/ subdirectory of the card. (You will need to use root/superuser access to have permission to modify the content of this directory on the SD card. Create that subdirectory if it doesn't already exist.) Edit the file /boot/armbianEnv.txt. (You will need to have root/superuser access to have permission to edit this file on the SD card.) If there's no line starting with "user_overlays=" in the file, then just add a new line to the end of the file: user_overlays= sun8i-r40-bananapi-m2-ultra-1bit-emmc Otherwise, if a line starting with "user_overlays=" is already present in the, then add a space to the end of that line's existing contents, and add sun8i-r40-bananapi-m2-ultra-1bit-emmc to the end of it.  
Note: I linked both the dtbo binary, and the dts source code which created it. Only the dtbo file is needed for a quick test; if you wish you could compile the dts file from source code and prove that my work is reproducible.