Your worry is well-founded but curable for the time being. Trails End Campground still has a few sites that can be reserved here (but they're going fast). I wouldn't count on getting a first-come site that weekend.
Trails End Campground reservations EDIT: I just realized that you want to camp inside the BWCA. Seagull is a very popular destination and I would want to be on the water long before 9 a.m. that weekend.