I'd like to be able to use the package manager for different channels of software. This could be beta, or unstable branches, or other kinds of constructs.

Semantic versioning (https://semver.org) has a specification we may follow for pre-release software.
Microsoft has channels.
Many open source projects have branches.

Note: Some packages support "side by side" install where a stable and a pre-release version may both be on a system at the same time. These are not considered release channels in the Windows Package Manager, but they are treated as different packages. Release channels are considered in the context of the Windows Package Manager for packages that do not support "side by side" installation.