I have used the PCA to drive a stepper motor. However, the algorithm was different than MK's. I used a lookup table in Code memory, but it was generated to produce a more linear (time-wise, rather than frequency-wise) ramp up and down. Also, the speed ramp was for the first 100 (and last 100) steps.

I put the PCA in the HSO mode, and used the port pin toggle feature. Each interrupt, I would use flags and variables to determine if it was in the first (or last) 100 steps, and also if it was in the positive or negative half of the cycle, before I would grab the value from the reload table. It sounds fairly simple, but it is actually about 350 lines of code to keep track of it all.

Make sure that you read up on the stepper that you are using. Many have guidelines regarding the step frequency min and max (and rate of change).