A bit old but for the record.
There's a provider called LoopBackAuth with a property called accessTokenId. You can get the access token doing something like this:
angular.module().controller('TestC', ['LoopBackAuth', function (LoopBackAuth) {
console.log(LoopBackAuth.accessTokenId);
}]);
Hope this helps someone. I had to dive into the code for the SDK in order to find this.