Maybe the question is whether the OP would be allowed to use a lounge with shower on the nonschengen side before clearing customs and immigration and checking in/checking the bag to the schengen destination. At AMS, arriving and departing passengers are mixed, so OP can surely walk to the door of any lounge in the nonschengen side upon arrival. The question is whether there's a *Gold nonschengen lounge with showers that would allow the OP to enter given that the departing flight is schengen.