> Is it possible to reverse the processs? from the V3Update.bin from Jinke, extract the files, update them and repackage them again.
Yes. Have a look at utils/mkv3update/scripts/firmware_unpack.sh
> but is it workable right now?
Not yet. You may build the firmware, but you'll get only bare system with X11 and without any graphical toolkit or X input method.
>have you planned a kind of an API for the developer?
It will be pretty standard Linux system, with X11, Enlightenment as GUI and any libraries you want (most of them are available from OpenEmbedded repository).