As Jody suggested I would implement unreal mode for ELKS. It is a simple
implementation to access additional memory. However, I would not target the
80286 processor but support 80386 and later processors instead. I do see no
advantage in supporting the 80286 processor instead of the 80386 and later
processors.
https://en.wikipedia.org/wiki/Unreal_mode
https://wiki.osdev.org/Unreal_Mode
Turning on unreal mode would just have to be implemented when booting. ELKS
will still run unchanged then. However, to make use of the additional memory
additional changes would be required.
Georg