Sorry for coming late to this thread.
I think the first thing to talk about regarding custom content on top of Clear Linux default bundles is why are they needed.
Most often things not existing is due to the team not having added them yet and in those cases we strive to add the content in a timely manner. The difficulty is when this isn’t timely enough for folks to get things done and for that I really do recommend docker images or flatpak like tooling if building it yourself and putting it in /usr/local doesn’t make sense.
For cases where it can’t be added to Clear Linux then we run into issues because an official user repository that Clear Linux hosts isn’t sufficiently different from Clear Linux publishing it as part of the distro proper. This makes an aur like repository more difficult because somebody outside of the project would need to maintain it.
The other issue with an AUR like system is content can conflict between packages and with the main distro proper in that setup as there is no coordination between the packages (and in some cases the conflict is nearly unavoidable anyway). To resolve this problem I have tried to develop a limited scope for folks to build (and share) 3rd-party content. This is found via man mixer-user-bundler and man swupd-3rd-party but it has a number of limitations at this point. Right now it only allows binaries to be exported to a user visable $PATH and they do not supersede binaries that Clear Linux provides. I have a roadmap for adding support for 3rd party kernels currently. We don’t have any roadmap for enabling 3rd party libraries to be exposed today.
That’s pretty much our current state and I definitely welcome feedback to our approach.