Well, KitKat is totally possible on locked BL. We don't need to change kernels for it, we "just" need to compile a module for every change in the kk kernel, and then insmod the modules before hijacking ramdisk. So, it's possible, but really time-consuming.
The best solution is to get kexec working on locked BL's.
There's a thread on xda about it, where delewer, dh.harald and others tried to make it work. Maybe you, @bagyusz
, should try? You showed us that you have great skills.
And, just to clarify, with kexec we will be prepared for Android 4.5, 4.6... and we won't need to make separate ROMs for locked and unlocked BLs. Also, MultiROM solutions would be possible.
Complete freedom and equality:P!
Sorry for my English!