The literature I know on imagery rehearsal therapy and PTSD-related nightmares is mixed. When I talk with my patients about possible psychotherapy options for this problem, the usual first-line recommendation is a course of Cognitive Processing of Trauma or Prolonged Exposure. For those patients who aren't ready for a trauma-focused therapy, Cognitive Behavior Therapy for Insomnia has reasonably good evidence in persons with PTSD.
The results on IRT are mixed and somewhat hard to interpret. You may be in the same position as I am: waiting for a good study on IRT in persons with PTSD so we'll know how to advise our patients.
Nightmares in PTSD is one of the clear-cut reasons to recommend medication. Prazosin is highly effective for this problem as shown by several very good placebo-controlled randomized trials.