what is your heat/wave?
If you are heat 1 & wave 1 and you get there early you can start immediately behind the pros. Just reach out and grab their buttocks. You could also pull a Seth Wold and sprint the first 5k so you can rub shoulders with everyone. It will be frowned upon by letsrunners but I'm sure its a blast.
I think a seed time of sub 2:45 or sub 2:40 will get you in heat 1 & wave 1. If you're slower than that you won't see them race day as you'll be standing behind anywhere from 1,000 to 40,000 other runners.
If you are not in heat 1 & wave 1 you may get lucky and see a pro at the expo or around the city. Some of the pros may also do autographs or whatever at the booths of their respective sponsors. If you are lucky you may see one or two African runners jogging along the charles 1 or 2 days prior to the marathon.