A couple thoughts, from the point of view of someone who has shot a lot of track and cross country, and a few road races, and probably raced just as many:
1) I don't think I've ever seen anyone trying to get people's attention at the finish line. It has never occurred to me to try it. If you do, you'll be unique in that. Let me know how it works! I'm genuinely intrigued.
2) I doubt you'll ever get everyone to look up. Be prepared for that.
3) I have always thought that the photos are for two main audiences: the runners, and the race organizers (promo material). The race organizers will certainly get enough from those who do look up to use for their promos for upcoming races. Thus, the only people losing out from shots of runners looking at watches, are those runners themselves. And they have only themselves to blame.
4) Setting this one apart, because it comes mostly from my point of view as a runner, not a photographer. Your real money shot is only at the finish line if it's a championship victory or a record time, AND the photo makes that apparent (clock in frame, breaking the tape, etc). Otherwise, people finishing a race look like crap! The whole goal of a race is to perfectly measure your energy output to be precisely spent when you cross the line. So people finishing are sprinting for their lives, form falling apart, and have death-mask grimaces on.
Instead, try for the first third of the race. Enough that it has thinned out a bit from the start, and everyone is in a good rhythm, but not yet tired. That's when you'll get people looking their best, having fun, and most likely to be alert and looking around (and therefore seeing you and smiling, waving, thumbs-up, etc).