The system is not that over-engineered, that you can throw away a bunch of the engineering and still have a working link. To look at a single line on a scope is often to see almost no signal at all. It's all noise. Only by looking at the signal differentially, does the data appear.
As for inductive coupling. Even if you used the entire differential signal, you still will fail, I think. The currents are quite low, which means your inductive pickup will need to be extremely sensitive. So sensitive, that I would anticipate the system noise of your inductive pickup to be on the same level as the signal you're trying to read, resulting in too much misread data to do anything with.
Plus, it's not worth it. There's no way to tell that another high impedance device has been added to the system, It won't change the impedance that some hypothetical tamper detection system would be able to measure, in any measurable way, so why not just add it using a direct connection?