--- So with a little bit of package editing within SimCity, 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 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.
Other things I have modded out with a quick change: Unlimited time to remain disconnected (won't get booted at 20 minutes, can now be disconnected "forever"). Population count now shows REAL figure, not the "artificially inflated" figure. My large cities have a population of about 15k now, not 100k :P
(NB: All modding work was done with the assistance of my good RL friend Weeble House - "MrWeebs" on Youtube)