The only difference between the SD card version and an eMMC version should really just be minor changes in u-boot that is the output of Amlogic signing tools and how it is written to the boot area. I’d say just blow out the Android partitioning altogether since that isn’t really necessary.
I will play around with the CoreELEC build system as well. I don’t think it is very complicated but I am up to my neck in work on upstream images.
Thanks for looking into this.