It would seem to me that a simple API that return an array of the API base URL and perhaps the common name of the sites that support the API would be valuable. Is this sensible?

EDIT:

This is what I want:

{
  "api" : [
    { "endpoint" : "api.stackoverflow.com",
      "name" : "StackOverflow"},
    { "endpoint" : "api.meta.stackoverflow.com",
      "name" : "Meta"},
    { "endpoint" : "api.SuperUser.com",
      "name" : "SuperUser"},
    { "endpoint" : "api.bobsyouruncle.com",
      "name" : "Bob's your Uncle SE"}
   ]
}