You didn't mention US! I had pretty good success upgrading last year for the short time that I was a Silver on US Airways (equivalent to 2P). Although their F product is not as refined as UA's, with boxed meals and plastic cups, US gives free, space-available upgrades to all elites so I can't complain too much if I'm not "paying" for the upgrade.

Good to know that you're still able to upgrade as a 2P when you choose to, however...and of course UA has an excellent E+ cabin to fall back on when you don't decide to upgrade or don't clear.