I've done it, and there's definitely a line. On out last trip, I regularly carried a small zippered make up bag (the fabric kind you get inside the bigger clinique bonus bags, about the size of my hand). Inside the bag I had my cellphone, a tiny folding hairbrush, an extra hair elastic, packet of advil, credit card, annual pass, and money. I wore regular jeans or capris and never had a problem walking in through the bagless line with this little bag in my pocket.
I also brought in an empty water bottle bag (a small nylon one with a carry strap), I just stuffed it in a pocket and carried in the water bottle by hand. Again, never had a problem going through the bagless line. Once inside the park we'd put the water bottle in and wear the bag around.
We did try a larger bag once, and that time we got stopped. It was one of those fabric backpacks that can be folded up into itself and zipped so it's like a pouch. I wore it in the pocket of my rain jacket one night, but it made a noticeable bulge and the guy at the bagless gate asked to see what was in my coat. I showed him the (empty) bag and he said "Aha!"and laughed, but waved us on through. I had the impression that we'd pushed the limits a bit but he didn't give us a hard time or send us to the other line. We never actually used the bigger bag, so we skipped it thereafter and stuck with the little ones.