While you can use X max for something else, there is simply no homing sequence for E axis, that is the main problem.
No mks 1.4 is avr based and I started developing on 32 bit platform first. I can not really say when I get first 8 bit boards running with it. It is planned once the feature set is complete. Until now there are already some changes in the hardware layer to support the flexible system, but I guess it should be not too hard. So once jam and autoleveling works I guess I will add the reminding hardwares I plan to support. On 8 bit this is avr2560 based boards like mks, but probably not the smaller version with only 128kb flash.