I agree that your best shot is working with breakaways. That is the one thing with actual numbers that would be relatively easy to track besides finishing time & position.

Look on CyclingNews and VeloNews for races that they covered with a live report. You can go back through those and track how many riders got in a break and their time gap vs. km until the finish. Then match that to when breakaways succeeded and when the race ended in a bunch sprint. It will take a lot of digging and you might not get perfect data for every race, but you'll at least have something to work with. Basically try to find anything you can to predict when a breakaway will succeed. Maybe for a stage race you could track each break rider's position on GC, as well.

You might also think about TTs and various time checks along the route, though I don't know if those are actually recorded and/or what you'd actually learn from it.