I would try this in your preprocess() method:
NavigationState navigationState = getNavigationState();
FilterState filters = navigationState.getFilterState();
I'm not sure this is going to work well, however. I believe this might modify the NavigationState for the entire page, not just the ResultsList. But it's worth experimenting with. You might need to get a little more clever and do work in the process() method, where you create another mdexRequest.