Please add the ~arm keyword after doing some manual testing. I don't expect you to connect an optical drive, but checking the makemkvcon CLI tool shows the help text and checking that the GUI launches would be enough. I was able to test the GUI on my arm64 board using eglfs.
> QT_QPA_PLATFORM=eglfs makemkv
Unable to check for sanity:
> package masked: media-video/makemkv-1.17.5, by keywords: -arm
Please ignore NATTkA, it doesn't know how to handle this case.
Bah, fine! I've temporarily adjusted KEYWORDS.
all arches done