Quick note: Don't coast in neutral at 100 mph, while I understand you want to save money that is basically putting everyone else around you at more risk if something were to go wrong. Yes, the ABS will still function as will ASC, etc . . but you GREATLY diminish the braking capabilities of the car (no engine braking and automatic downshifts).
The engine should go into fuel cut mode after only a few seconds while coasting without using the gas. So, while it will be engine breaking, you will not be using any gas at that time.
Now that all of that is out of the way . . . NO. You can shift into Neutral when going 10, 50, 100 mph and it will not hurt your DSG. As long as you re-engage the DSG WITHOUT pressing on the gas, it should automatically read the current speed and choose the correct gear to shift back into before you press on the gas.