You could post the event with a dummy Game Object. In Unreal Wwise integration, there's one registered already called DUMMY_GAMEOBJ.
If you're not using that, you could register your own.
const AkGameObjectID DummyGameObject = 1;
Wherever you Init your Audio engine stuff:
//// Register the dummy game object. It is used for the 2D sound (UI etc...).
AK::SoundEngine::RegisterGameObj(DummyGameObject, "Global");
If that doesn't work, you could set the scaling factor to 0
AK::SoundEngine::SetScalingFactor(DummyGameObject, 0.0f);
The when you want to play the event in 2D, you can call PostEvent with the DummyGameObject instead of whatever object you'd normally use.