E.g. 

expect().body("root.child", equalTo("x")).when("/jsonp?callback=y");

But perhaps it's better/easier to do something like:

expect().jsonp("callback", "y").body("root.child", 
equalTo("x")).when().get("/jsonp");

or:

expect().jsonp("callback").body("root.child", 
equalTo("x")).when().get("/jsonp"); 

which should generate a random method-name.

It should also be possible to set it as default:

RestAssured.expectedParser = jsonp("callback", "y");
RestAssured.expectedParser = jsonp("callback");
RestAssured.expectedParser = auto(); // Default

There should also be a JsonpPath that simply extends JsonPath and removes 
method call.