After searching the web I did find several references on the subject. In general, placing an RPZ in a pit is not recommended. It is possible to place an RPZ given proper drainage. In several cases drainage must be to daylight i.e., a simple sump is not good enough.
In my town, Hollis, NH there is no regulation regarding placing an RPZ in-ground. In fact, there is no requirement that a backflow preventer is necessary at all. However, what may be required somewhere else probably makes good common sense everywhere elese.
To summarize, I plan on using an RPZ that is installed above-ground. Instead of an expensive enclosure, perhaps I will build a "dog house" like enclosure to match the house.
-WH