A clinical case report is a good way to disseminate new knowledge gained from clinical practice. Medical practitioners often come across patient cases that are different or unusual such as a previously unknown condition, a complication of a known disease, an unusual side effect or adverse response to a mode of treatment, or a new approach to a common medical condition. These are generally published in the form of clinical case reports to make the medical community aware of this new knowledge. Other researchers can then conduct further studies based on this to advance knowledge on that topic.
You can definitely publish a case report detailing how the use of the FDA recommended treatment has helped in the other medical condition. There is nothing unethical in this. Of course, you should mention that the mode of treatment is recommended by the FDA for a different condition and explain why you used it to treat the other condition. This can, as you have suggested, be a first step to get the medical community thinking about this mode of treatment.