I saw a sign of the local environment protection foundation (it Fryske Gea) indicating entering the salt-marsh area is allowed outside the bird breeding season (concrete: outside 15 March - 15 July). That is good news! My impressions was it was not allowed anyway. So, my plan is to see if a hike is possible through this area.
Also, De Zwarte Haan is a crossroad of two ultra long distance trails in Europa, namely the Jabikspaad, start of trail to Santiago de Compostela in Spain (more info), and the Europe coast trail E9 (more info). Being a crossroad, together with the fact this is a spot at the salt marshes of UNESCO Wadden Sea ànd its local, cultural history of the 'mud workers', makes it a unique point.
Interested in walking this area? Read my blogs and website Frisia Coast Trail.