You need to contact Rockchip electronics, everything you need to know is here in this thread:-
This is something many have tried to do without success, brands like Teclast and Onda and now Microsoft will have this SDK because they make upgrades to the basic Rockchip firmware, but I suspect it comes at a price.
The firmware itself is coded in C and there is a datasheet in the download section.
The code I believe is based on the ZSP400 Digital Signal Processor
Architecture and I do have a technical manual with it's instruction set.
The problem is the code is packed into the .rfw file, signed with a 13 byte MD5 checksum at the end and is unpacked by consumerUpdate during the flashing process, so you would need to hack consumerUpdate first.
I wish you the best of luck!