You're right, I think it already is a short day hike. To get an early start, you can camp at Hancock or Big Rock camprgrounds, very close to the trailhead. If you insist on camping along the trail, there is plenty of flat ground along the Lincoln Woods trail or the bottom 1/3mi of the Osseo trial, but beyond that the ground gets slopey, rocky, and heavily wooded. Given the tiny mileage difference, I'd prefer to camp at Hancock and not worry about finding a site.
There's always Liberty Spring tentsite but it sounds like you don't want to do the elevation gain with a tent on your back, never mind the fact that you'd have further to travel to get there from Lincoln Woods. That tentsite might be more attractive if you use the Liberty Spring trail instead of Osseo.
I do worry about the idea that you don't think you can handle an eleven-mile day hike but you want to carry camping gear on your back (and you're apparently heading out solo). Have you tried some easier day trips first? (Cardigan, Monadnock, Pemigewasset, Chocorua...)