You could try this:
- Make a slide with the menu button, not clickable
- Second slide: copy of the first one, but menu button clickable to open nav points
- Set transition of first slide to 'Advance Slide: After 00:00' (= go to Transitions tab, uncheck On Mouse Click and check After...)
- Make sure that when you go back from any slide to the menu slide, you go to the first one and not the second one