Values are returned by reference in Objective-C by passing a pointer to memory where the result should be stored, just like in the C language. The Objective-C interface in Lisp works similarly, using the standard FLI constructs for this.
For example, an Objective-C method declared as
- (void)getValueInto:(int *)result;
might called from Objective-C like this:
int getResult(MyObject *object)
The equivalent call from Lisp can be made like this:
(defun get-result (object)
(fli:with-dynamic-foreign-objects ((result-value :int))
(objc:invoke object "getValueInto:" result-value)
The same technique applies to in/out arguments, but adding code to initialize the dynamic foreign object before calling the method.