How to make a decoder or reencoder plugin?

[brunoais]

Intro + Background knowledge + Justifications

In my unfortunate situation, I have a server with many HVEC encoded videos which I never knew were supposed to be shown on users browsers... But here we are.
Due to the expected audience and avoid running the server to the ground with reencodes and with the "NO" towards having two versions of the same videos and reencoding to a format that takes way more space is not acceptable...
I'm trying to provide an option to decode or reencode video on the browser itself. In addition to use fmp4.

For what appears like, safari supports HVEC, so I wouldn't need to act on that audience. However, there will also be Android and windows audience (for the majority).

I've been using video.js for quite some time, I've wrote a plugin for it but I never wrote a plugin for reencoding (or decoding).

The real question

What manuals are there on how to tap into the byte streams so I can apply reencoding (or decoding) of HVEC?
I will be using either HLS /DASH and with fmp4 as the container to facilitate the caching on the edge servers.

What I tried to look at

In a hack way, it appears like the best place to edit code would be the VHS plugin, wherever it uses the XMLHTTPRequest to get the response, hijack the response and, if it's HVEC, reencode into AVC and allow the rest of VHS to run normally.
Because VHS is now part of video.js... I'm not sure how that would be done.

However, I'd like to try not being that hacky and try to do it as a proper plugin instead. Are there means to intercept the video bytes and do a reencode switch?

Thanks in advance.