I have the following use case at: https://codereview.qt-project.org/c/qt/qtbase/+/675552/20

Build Qt, with a CMake built from source, on all default platform configs, in a nightly configuration, without having to duplicate all the platform configs to add a single BuildFromSource feature to each of them, and without affecting how regular builds work in CI.

One way could be to add two new yaml properties ExtraEnvironmentVariables and ExtraFeatures to any platform.yaml file, somewhere next to the Include or Configurations sections.