BLE, HID descriptor for a (Gameboy like) 8 buttons gamepad?

Well it seems the limitation comes from the module itself, in particular the method tap() which sends press and release successively...

exports.tap = function(keyCode, modifiers, callback) {
  NRF.sendHIDReport([modifiers,0,keyCode,0,0,0,0,0], function() {
    NRF.sendHIDReport([0,0,0,0,0,0,0,0], function() {
      if (callback) callback();
    });    
  });
};