Appreciate the response. I'm amused but unfortunately no closer to finding a good answer.
The PE ethics and code of conduct describes how, as long as you can justify (possibly in court) that you have engineering level expertise in a particular discipline, then you would be fine stamping a document.
For example, I took and passed the Computer Engineering PE exam, which is approximately 40% hardware, 40% software, and 20% networks. However, the bulk (like 90%) of my experience is in the specific area of software and computer security (with 10% on computer systems hardware and logic design). Even though I could prove I meet the educational requirements for engineering a communications transmission station, I never worked in the field on such transmitters and would be VERY hesitant in stamping a design document for one.
However, I do know of Power engineers who never really worked on computer systems, computer networks or logic design, who have stamped computer hardware layouts and network topology designs. If something goes wrong with one of those I wonder if they would have any trouble explaining the details of exactly what they stamped.
In the end, I think if its close, then the assumption is that a PE would not stamp something that is at leas not in their general field - such as an electrical engineer stamping plans for highway bridge, or a mechanical engineer stamping an environmental plan - but you never know.