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"}
]
}