Equally interesting is the possibility of running non-Endless OS images on the Endless Mini. That has the same issues though: the devtree, kernel, boot loader, etc. are very specific to the Mini even though the board is an ODroid-C1 (Amlogic Meson). That might be an easier project to start with though.
Endless provides their changes from upstream in their repos so you can see what modifications they’ve made in the kernel, X.org, etc. That should also help in porting other distros to work on their ARM hardware.