Is there a version of $getJSON that doesn’t use a call back?

Looking at the jQuery source code, this is all $.getJSON does:

getJSON: function( url, data, callback ) {
    return jQuery.get(url, data, callback, "json");
},

And this is all $.get does:

get: function( url, data, callback, type ) {
    // shift arguments if data argument was omitted
    if ( jQuery.isFunction( data ) ) {
        callback = data;
        data = null;
    }

    return jQuery.ajax({
        type: "GET",
        url: url,
        data: data,
        success: callback,
        dataType: type
    });
},

No black magic there. Since you need to customize stuff other than the basic $.getJSON functionality, you can just use the low-level $.ajax function and pass the async option as false:

$.ajax({
    type: 'GET',
    url: 'whatever',
    dataType: 'json',
    success: function() { },
    data: {},
    async: false
});