Is there a term for a group of people arranged in such a way as to appear from an aerial vantage as a specific shape, picture or word?
The term you're looking for is crowd formation.
You will also find this term returns an acceptable range of pictures when searching.
The phrase "tableau vivant" which is French for "living picture "is defined in Wikipedia as
The term describes a striking group of suitably costumed actors or artist's models, carefully posed and often theatrically lit. Throughout the duration of the display, the people shown do not speak or move. The approach thus marries the art forms of the stage with those of painting/photography, and as such it has been of interest to modern photographers.
The phrase is often shortened to "tableau".
Originally meant as a formation to be viewed from the front, perhaps "aerial tableau" would address your need.
The Wikipedia article refers to the pictures as performed group photography or "living photographs" (with quotes).
Indeed, a Google Image Search for "performed group photography" returns lots of relevant results, and a textual search returns more relevant results than one for "crowd formation".
I don't know, but I would suggest a "busby", after the master of that particular art ("berkeley" probably has unwanted connotations). See here... http://en.wikipedia.org/wiki/Busby_Berkeley