One more update from the original poster
OK, so here I am at MSP on my layover en route to MSN. I thought I'd check on that originally planned flight from SEA-ORD . . . DL 1087 . . .well, guess what? It departed SEA just ten minutes late. But it got diverted. And guess where it got diverted? Yup. Madison.
The thing about diverted flights is sometimes they have passengers stay on the plane during the entire diversion, and the plane doesn't even park at a gate. If it's a longer diversion, sometimes they'll park at a gate and let passengers deplane into the terminal (I spent a half day in the Peoria airport one time when an ORD-bound UA flight got sent there). In the latter case, I've seen cases where passengers for whom the diversion airport was the final destination counted their blessings and walked away. However, that would not have worked in my case, because I have a checked bag.