Why don't DO guidelines have examples?
RTCA documents are not FAA documents, although the FAA is usually a member of the Special Committee that creates the documents. If an example is to be added, it is the decision of the SC, not of the FAA. 

The reason that no examples are given is that the recommendation is clear by itself. A good V&amp;V engineer will read this recommendation and use his skill, experience and creativity to find an other acceptable means to validate the software requirements that can not be validated in a realistic environment. This alternative means is then documented in the SVP.

Giving an example is not helpful at all, since the solution will depend completely on the software under test and the nature of the requirements.

If the hardware is not available, an emulator may be the solution.

In other cases artificial test inputs have to be generated, because failing a test in a realistic environment may prove to be catastrophic.