It’s done! Please see this post for details and downloads.
Also, I’ve renamed the repo I was using for arm64 releases as it now covers building all ‘MKVToolNix for Mac’ packages and also includes the tools used to create universal binaries. It’s now called package-mkvtoolnix-for-mac and release 1.3 is available including the MKVToolNix 98.0 revision 1 package artefacts 
The next task is to start on the Homebrew cask code so it can serve the right binary for each user