I also noticed how fully booked the backpacking trails were leading up to the long weekend. (Eastern Pines too, by the way.) I can't directly answer your question, since I was canoeing. But I can attest that there were real live backpackers camped on Harness Lake, which is the Highland Trail's farthest lake from the trailhead. Of the two backpacking campsites that I could basically see from my canoeing campsite, both were occupied on the Saturday night and one on the Sunday night, and there seemed to be a decent amount of backpacker traffic on the trail when I day hiked on it on the Sunday.
Granted, the WU trail goes farther away from access points than the Highland. But the campsite bookings weren't fully vacant for the Friday, were they? Surely some people started before the weekend, and took more than one day to get to those far-out lakes?