I am not familiar with IAH, however, the LAX TBIT arrival level (temporarily closed) TSA security checkpoint is accessible from the curb - if you know how to find it - in addition to passengers who are exiting the immigration and customs and connecting to other flights. In Miami, based upon my experience, access to the TSA checkpoint at the exit from the CBP facility is restricted to connecting passengers who arrived on international flights and cleared immigration and customs inspections in Miami. The LAX transfers security checkpoint does not have the new CT scanners that Blumie mentioned in his post.