There should be no problem triggering on the rising edge of the signal. That part should be easy. What isn't easy is triggering on the beginning of the pattern. Perhaps the pulse width is a minimum, rising to a maximum and diminishing to a minimum and you want to trigger at the beginning of the cycle.
I don't know how you are timing your PWM but if I were doing something like that with a microcontroller, I would generate a trigger pulse with one of the IO pins.
Chapter 5 (page 5-8) in the User Manual talks about triggering on a pulse of a certain minimum width. So, presumably, you can trigger on the wide pulse in the sequence and then move the image around to get the first pulse on the left edge.
In fact, spend some quality time with Chapter 5. The DS1054Z, like most DSOs, has an enormous number of trigger options.