Solarmass
From an official Solus packaging perspective, there's obviously the option to build only the main kodi distribution and the kodi-platform package.
This would allow users to follow a guide in order to build the binary add-ons from e.g a from-source package.yml via solbuild and have said locally-built package install to /usr/local.
However, I'm not sold on this option as it will break every time there is a kodi rebuild.
But to answer your actual question:
Right now, there's no simple way to test alpha v19 on Solus unless you want to build the package yourself -- either directly from source or via a locally written package.yml, which builds to an installable .eopkg using solbuild configured with a local repository.
