Check out the above image from a company called Trakus. They place a small transmitter inside of the saddle cloth prior to the race, then track the horse around the track in real time. Very cool. I've also seen them include speeds in mph during the race as well.
Using heart rate as a measure of exercise intensity, imagine how including that data could be helpful to the handicapper:
How intense is the warm up? Is the splenic contraction elicited?
How quickly can a horse reach his/her maximum HR leaving the gate?
During the gallop out, who recovers the fastest to below 100bpm?
What is the maximum speed? How long is it held?
If you could add this physiological data to other past performance info and note trends over time, you could pinpoint quite a bit about each individual's actual athletic ability - and note when a peak, or a bounce, is likely to happen.