I know nothing about how it works but here's my guess.
When it selects what map to choose doesn't it load it into the game memory? If you have a program that reads the games memory, wouldn't blizzard be able to scan and dectect it.
Any program that does this would probably have to inject a DLL even if it is clientside, so the Warden would own it anyway. It wouldn't be hard to make a maphack, the only problem is making it undetectable.