gistfile1.js

Y.Echofin.Modal.async('/path', function(dlg) { 
    Y.delegate("click", function(e) {
        //from in here you can still call stuff from outside the .async 
        //call thanks to closures, so go nuts!
    }, dlg, "a.img");
}, this);

gistfile2.js

function(path, callback, scope) {
    //do some stuff
    //on success of the ajax & insertion
    callback.call(scope, dialog_reference);
}