At long last (I think) I’ve arrived at a conclusive answer.
The 20MB value is an MFE specific value in the frontend-app-authoring component.
In the file ModalDropzone.jsx#L18 the value is read from constants.js#L55:
export const UPLOAD_FILE_MAX_SIZE = 20 * 1024 * 1024; // 100mb
here the comment says 100mb even though the value hard-coded here is 20mb, I guess maintainer forgot to update the comment 
I still haven’t figured out if/how this is patchable with plugins, but for now at least I’ve just disabled the Django waffle flag contentstore.new_studio_mfe.use_new_files_uploads_page which reverts to the old version of the file uploads page, this correctly displays the value of MAX_ASSET_UPLOAD_FILE_SIZE_IN_MB which is in my old patch.
It would probably be ideal if UPLOAD_FILE_MAX_SIZE could read the value of MAX_ASSET_UPLOAD_FILE_SIZE_IN_MB instead of fragmenting values across multiple places, but this is a bit beyond my scope…