Interesting you should mention that. Right around the time you implemented multi-project pipeline visualization, we built a bot that listens for GitLab build completion events, hits the (undocumented /unsupported) internal GitLab global code search ElasticSearch index, finds downstream dependencies of the completed build (as declared in setup.py/package.json), and triggers their builds as well.. Taking full advantage of your nifty visualization.
So I think the feature we'd want to make that easier is an actual global code search API!