No, I didn't. But frankly, I wouldn't think that anyone at the dealership would really have the engineering insight to know whether the brake should be applied at "speed".

I've read that application of the parking brake at speed could be dangerous, prone to causing fishtailing and a spinout if both rear wheels are made to lock up at the same time (by yanking up the parking brake handle) that the front brakes (actually, the "service" brakes at all four wheels) are applied -- this being the natural reaction of someone trying to stop the car.

I've "experimented" with this at times -- e.g., when I suddenly see a cop in my rear view mirror and want to slow down without letting him see brake lights reveal my guilty conscience! A slow, gradual application of the parking brake handle can be an effective adjunct to merely lifting off the gas pedal in such a situation -- but the application of the parking brake should be judicious, as you certainly don't want the cop to see a swerve and smoke from a locked rear wheel (a la the drama seen on unladen tractor trailers sometimes).
I have also experimented (on a deserted road at slower speeds) with stronger (yet still careful) application of the parking brake *without* application of the service brakes, and even this way have felt the onset of such fishtailing -- a little unnerving!
This should underscore that we have to have a lot of trust in the condition of our service brakes, and should maintain them properly :-).

But the bottom line, viz this topic, is that it's inconceivable that application of the "electronic" pushbutton system in the S80 could be moderated so delicately. So I wouldn't think of using it as such. And I wouldn't trust the answer/opinion of anyone at a dealership level. It would, however, be something worthwhile to ask of an engineer at Volvo headquarters.

Meanwhile, I would only consider it as a parking brake, to hold the car should the parking sprag in the tranny fail.