I have seen either film or still photos of people standing along the motorcade route on the way to the NASA building. If you haven't already tried it, you might look on the JFK library website for film shot by White House photographers on that day.
Let me also suggest you consult with two people.
J.R Gonzalez blogs about Houston history for the Houston Chronicle, and he has access to the Chronicle's photo archive. He may know the answer to your questions, or he may be interested in doing some research that he can use in his blog.
Another possible resource is a man named Bob Gomel. He's a former Life magazine photographer who now lives in Houston. He covered Kennedy's speech at Rice and he appears in some photos that were taken that day. I suspect he accompanied JFK's motorcade that day. His email address is gomelbob@gmail.com. Feel free to tell him I referred you to him.
Good luck.
Doug Miller
