Good new libav implements hardware video encoding with OpenMax IL implementation !
See : http://www.phoronix.com/scan.php?page=news_item&px=Libav-OMX-H264-MPEG4
I github trunck get and compile it on my RPI 3 under ubuntu mate with openmax il hardware h264 encoding
I am not very good with video encoding I will make some tests next week and post here !
I hope ffmpeg will implement it soon too !
Stay tune !