We could implement some switch, but as far as I can see, beta pre-releases are used only very rarely, or it is something new at Emby? E.g. v4.9 did have two pre-releases, but v4.8 and v4.7 did not. Currently, the beta provided on their download page is older than the stable release:
Currently we pull the latest release from https://api.github.com/repos/MediaBrowser/Emby.Releases/releases/latest, which excludes pre-releases (notation on GitHub, provided as “BETA” on their download page). We could implement some option to pull from https://api.github.com/repos/MediaBrowser/Emby.Releases/releases instead, and take the first/latest one there. This includes pre-releases, but at time of writing would still take the stable release, as it is the latest one.
But not sure how many users would really want to do this, and doing this manually is very easy as well, following the download + dpkg -i instructions from their download page.