[Libav-user] Bitstream filter for watermarking

[Victor dMdB]

Hi,

How challenging would it be to create a bitstream filter which would add
a watermark/overlay to each frame?

I'm aware this can be done with libavfilter, but that requires
decoding/encoding. So my question is if this could be done at bitstream
level to make the process faster and/or more efficient. 
We would assume we know what the encoding format is.