You can get to the L concourse from all concourses in T1-T3. They're all connected inside security though it may be a long walk. I'd sooner do that than take chances with security. I also wouldn't believe that the TSA clerks would let you through the employee lane regardless of wht the USO employee said...on the other hand, if you have time I suppose you could take up the fight, armed with the cell number of the USO employee.