One of the problems preventing "normal" handling of community PRs to the Science Pipelines code base is that lsst_build and therefore Jenkins cannot clone from forks. As a result it is difficult or impossible to run CI on a community-contributed PR without creating a corresponding branch in the unforked repo.
There is some residual code that handles repository patterns that could be used to override the repos.yaml repository URLs, but unfortunately it is considered after repos.yaml.
Another alternative would be to copy the repos.yaml in the lsstsw directory, append the fork(s), and use that. Later entries should override earlier ones. This appears to be feasible to do within lsstsw/rebuild.