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Nice thread, COcyclist!
I also have some experience with TDI's. Learned to drive in a VW Transporter 2.5 TDI 5 cilinder, and now I have my own car with a 1.2 TDI 3 cilinder.
Big difference between the engines (big vs small) is that pulse and glide in the 2.5 TDI wasn't very effective, the engine had great breaking power because of internal friction, and coasting was possible but only at low speeds or steep descents because of the car's dramatic aerodynamic drag - it was a van.
The small TDI however has very little braking power. In fact, at small descents (2-3%) the car won't lose speed at all when releasing throttle. At decents of 5% and more it will even accelerate and I have to use the AC and/or downshift to generate more braking.
But I'm not using pulse and glide or coasting to get good mileage (yet).
On flat roads I will use cruise control and maintain steady speeds.
In hilly landschapes I'm never using cruise control, but I'm guided by the slopes and descents. So downhill I will gain some speed to maintain it below, and lose some speed going back uphill. For example, down the hill I'm speeding at 75 mph, and I'll reach the top at 55 mph. Always staying in highest gear, as you said, the TDI has a lot of torque and won't force you to downshift.
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