Now I understand what you're saying about the DME "trick", I don't understand how it actually works. What this means is that the DME would read negative if you're between the threshold and the antenna. Wouldn't that mean it would have to send its response pulse before it received the interrogation?
It works because a "normal" DME reply includes a fixed standard delay to allow for ground station processing. By reducing the delay you can "subtract" some distance, and make the DME appear closer than it really is. By putting the ground station equidistant to the two thresholds and subtracting delay equal to half the length of the runway, the DME reads "zero" at the threshold. And yes, after you have landed you will be into "negative" territory - next time you are at an airport with such an installation, take a look and you will notice that the DME will lose lock when you are closer to the ground station than half the runway length.