Cruise control and AT
 

Anyone ever come across automatics with cruise control that change down if they overspeed on a descent?

My Escape (6 speed auto) will downshift if need be (its flat as a pancake here, I really noticed this last summer on a long road trip)

My 2001 Volvo S40 would change down to maintain speed when climbing, as did my Vauxhalls (GM) before that, but when descending they would just stay in top, and if the speed exceeded that set, you had to brake to control it.
I know that some more upmarket models automatically apply a bit of brake if the cruise control overspeeds, then reapplies throttle when necessary.
I have never driven an automatic that downshifts to control overspeed. Most automatics I have driven (all hydraulic transmission - Borg Warner style) do not have a great deal of engine braking, anyway.

I drive a 2015 Audi Q5. My cruise control applies the brakes if I overspeed. No downshift, though.

For hypermiling, I set cruise to 50 mph. When I'm descending a hill (and speed limit is higher than 50 MPH), I disengage the cruise. Fuel consumption goes to an indicated zero, the car speeds up, I gain momentum. As that excess energy bleeds off (with zero fuel consumptions) and I approach 50 MPH, I re-engage cruise.

The best fuel economy I ever got using this technique was 4.54 L/100 km (=51.8 US MPG = 62.2 Imp MPG) across 528 miles. Pretty awesome, for a 3.0L diesel!

I've read the Prius makes use of regenerative braking to slow down. I'm guessing the use of some braking method by the cruise control happened with the introduction of the radar systems.

Regenerative braking is another thing again. All your hybrids and EVs use regen braking to harvest energy, just as F1 cars to with KERS (or as it is known now ERS). The new Nissan Leaf can be put in ePedal mode where you only need to brake for an emergency stop. Everything else is controlled by the throttle pedal.
Adaptive Cruise Control uses the radar to control the distance from the vehicle in front. The style of cruise control I am talking about is the "old style". It came in two flavours. The expensive variety used the set point to apply throttle below set speed and brakes above, give or take a certain amount of hysteresis. The poor man's cruise control (the only kind I have ever driven) uses the set point to apply throttle below the set speed and gravity(!) above. Some people tell me there is a third variety, a variation of the poor man's, that changes down if the set speed is exceeded, to apply some engine braking.
Anyone any experience of this type?

No, even the Lincolns we had were equipped with the 'poor man's' type.

Currently, I don't trust the cruise control to do better than me for fuel economy on hills. So can't say how it behaves there. Only use it during long trips on the interstates. The sections with slopes that could lead to speed increase usually have too much traffic for the CC to be on, and then air resistance would help keep any accelerations in check for the speeds traffic is at.

I used to use CC all the time, but I wasn't worried about FE then. Switching it off on hills certainly helps as you can drive much smarter than CC does.
My car is manual, and doesn't have CC - in fact, I don't see much point to CC unless car is automatic!

I'd disagree with that, I use mine regularly and I hardly drive on motorways. When I drove to Edinburgh, I had a 144 mile stretch of motorway that required no speed changes or exiting at all. I can imagine foot cramp would set in on those lengths.

For journeys like that a manual and CC is fine. On the automatic, I used mine all the time. Even in 20 mph zones (just). I used it to control my speed, to prevent plod taking my photo! In a 30 the automatic would change up and down, to suit itself, and I would steer, safe in the knowledge I wouldn't speed.
Modern cars have the speed limiter option. One of the works vans I used had that and it was great. Citroen Berlingo to be precise.