The slice approach should be pretty readily doable today with e.g. libx264 (which you can force exactly N rectangular (full-row) slices with, using i_slice_count), and calculating N based on the resolution of the eleven vertically and horziontally stitched clips and their boundaries. (And with VP9 using some craft tile-column/tile-rows setup, maybe...)
This is assuming the videos are stitched pre-encode, of course... From the post, it almost sounds as if the idea would be to stitch independent H.264 streams into a new unified one using slice mangling + slices... which would be pretty crazy stuff.
(As a side note, it's a shame flexible macroblock ordering is only in the baseline and extended profiles... I still don't understand that decision at all.)
EDIT: Dawned on me that the hard part is on the client/decode side. D'oh.
This is assuming the videos are stitched pre-encode, of course... From the post, it almost sounds as if the idea would be to stitch independent H.264 streams into a new unified one using slice mangling + slices... which would be pretty crazy stuff.
(As a side note, it's a shame flexible macroblock ordering is only in the baseline and extended profiles... I still don't understand that decision at all.)
EDIT: Dawned on me that the hard part is on the client/decode side. D'oh.