How to embed additional metadata to a video file in libavcodec (FFmpeg C source code fork)

Question

I'm extending an existing codec (Hap) inside ffmpeg library, adding some additional stuff to it (modifying FFmpeg code in my own fork of it)

One of the things I'd like to do is to store a per-file maximum encoded packet size in some global header/metadata so I can use it later while doing decoding (generally, the metadata/info to store can be arbitrary, size is just what I currently need for some specialized decoder I'm writing).

I'm modifying the ffmpeg code file https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/hapenc.c

EDIT: What I'd like to achieve

• Compute some numeric value during the encoding (namely, the maximum encoded frame size for the video - it's needed for a specialized fast decoder to not read all the frames beforehand) - that is done already
• Store that value in the video file metadata (looks like it should be done on container level)

My initial incorrect attempt: But I don't currently see any possibility to save any info outside of a single packet being produced by hap_encode function and written to AVPacket *pkt parameter.

1. Set dict parameter on AVCodecContext *avctx's priv_data:

    av_opt_set_int(avctx->priv_data, "max_compressed_frame_size", <size>, 0);
    av_opt_set(avctx->priv_data, "max_compressed_frame_size", "<size_str>", 0);

Later, while decoding av_opt_get/av_opt_get_int report non-existing key.

2. Set various AVCodecContext *avctx fields:

    avctx->flags2 = <size>;
    avctx->extradata_size = <size>;
    avctx->ticks_per_frame = <size>;
    avctx->delay = <size>;

They all also seem to be not preserved when doing decoding later (I get just default values from decoder's AVCodecContext)