For anyone who is interested there is a open source OpenOCD / GDB WIP debug solution available for HiKey.
I've created a wiki page here https://github.com/96boards/documentation/wiki/JTAG-on-HiKey
Which documents the hardware setup, where you can get the code, and a overview of what is currently working.
Thanks to Philip Attfield who has already provided some patches to overcome several issues I was facing with read_memory and GDB config.
Please feel free to update the wiki page with any additional info you find out (thanks to Dan Thompson for already doing this), and patches making OpenOCD support for HiKey even better are most welcome :)