I get the FHANA magazine every couple of months, and they usually have some small bit about Friesians being used as police horses and teams in the mountains. I don't have the magazine right in front of me, but there are 2 purebred Friesians on some search-and-rescue team in the mountains of Colorado, I think. They told a story of how the Friesians were able to keep up just fine with the 2 quarter horses on the mountain sideslopes and ravines, and had a lot of stamina. I personally think you could get a Friesian there with conditioning work, just like any other horse (my filly's dam could jog around with a cart all day- though she didn't have that high action trot- and never tire). Obviously though, it will depend on the horse.
And not all Friesians have the high action trot. The German registry breeds more for dressage movements, and doesn't like that trot as much (or so I've been told).