Alright HERE I am!
Although we have not had Standardbred horses we HAVE had
Racking Horses NRHA and Tennessee Walking Horses.
There should be no surprise that a Standardbred can be a fine racking horse. Remember that both the American Saddlebred and the Tennessee Walking Horse come from Standardbred roots!!
You are right about that gait ... super cool. The Racking Horse that we had for a few years was double registered TWH. Her name was Lady and ... like wow man ... that witch could FLY!!
Part of the problem that people encounter with teaching a Racking Horse to canter is that the canter is a gait begun by the hind foot, followed by the next diagonal pair and then ending on the front foot diagonal to the starting foot called the "lead" foot. If you look at those pics the horses are so extended during the rack it is very difficult for them to gather up and change their leg pattern.
A good working canter with balance and athletic motion is even more difficult for a Walking horse, since the running walk is a broken pacing gait and the canter is a diagonal gait. That is why, when showing, both Walking horses and Saddlebreds usually come to a walk before picking up the canter.
A good trainer will insist that a horse be able to collect as well as extend in ALL gaits. (A nice extended walk, which you never see, is great for the horse and a good long day working gait.) A good trainer will teach a horse to move with his back end under himself. This makes them light in the shoulder AND light in the mouth.
If your daughter is planning on doing any jumping, plenty of work on extension and collection (at all speeds) will be very beneficial. Also work on the haunches (such as a turn on the hindquarters or pivot) and turn on the forehand (around the front legs) will be helpful. You want to be in complete control of the horses whole body and able to shift his weight where you need it - with the touch of a "button". At that point he will be ready for ANY athletic activity you choose.
My "mane" mare when I was young was basically dressage trained like this. She was a very TB Quarter Horse. We competed in Open Jumping, Reining, and the race games (Barrels, Poles, Flags) as well as Equitation Classes. When you can control their motion they are versatile!
We also had a Tennessee Walking Horse stallion, Treasure the Sun. I did a majority of the training on him as well. He could perform a flat walk, natural running walk, square trot, fox trot, rack and canter. He was inspected and then registered as a
foundation Fox Trotter after he preformed his fox trot for the inspector. He could also preform sliding stops and rollbacks and had a glorious athletic canter.
I had so many people tell me he is a "Poco Bueno" horse. 
