There have been extensive threads on the topic of general purpose network access. Let's keep that conversation separate, as the ask in this thread is explicitly not to grant network functionality to non-catalog games, and instead to enable use of the local-only high score caching by anyone.
Check out pdPortal (Discord, Github) for some ideas on how to do level sharing without network access. The Spilled Mushrooms and Trackminia discord channels (among others, I'm sure) have also both recently been discussing approaches.