To update:
a) After some basic investigation we found that swrast currently has hard requirements for X11, and our long-term plan (LE10) will see us drop all support for X11 (AMD and Intel move from X11/GL to a KMS/GBM + VAAPI (GLES) back end, and nVidia is probably discontinued) so we're not interested in pursuing that option much further.
b) No visible changes in Libhybris status, and this direction is still firmly seen as a hack by LE staff and Kodi staff.
c) The mesa driver is now capable of rendering on T860 hardware (used with the RK3399 boards and very similar to T820) in addition to T760 (used on RK3268) but the corresponding kernel driver is still at the earliest stages of being written. Great momentum building but a mountain of code to climb before we have anything useable.
In summary.. don't hold your breath for S912 mainline support.