So with a little bit of package editing within SimCity thanks to the modding guide posted to Reddit earlier, and a little playing about in the code, it's possible to enable debug mode. I linked the activation to the "Help Center" button in the main menu for ease. Most developer debug features are disabled without having an actual developer's build (they have terraforming tools etc. available in the full developer build!), but a few things do still work - including editing the main highways.
Not only that - but you can edit the highways ANYWHERE - even outside of your city boundary... and even if you quit the game and log back in later, it's all saved safely on the server.
This shows that highway editing will be easily possible, AND that editing outside of the artificially small city boundaries should be very viable too, potentially (since you can place roads and rails that snap anywhere on-screen).
Edit: And yes I also modded out the disconnect timer (can now play "offline" indefinitely - but no saves/syncs or region related stuff, not yet anyway... but the simulation can carry on with no connection indefinitely). And I modded out the "fluffed population count", just shows the real population count now. Both very minor/easy tweaks.