Thanks for giving me a word to apply to the concept, Ari. I've been down that line myself recently and didn't have the vocabulary for it.
A couple more points that might be worth bringing in to this discussion:
1) Fallout or Consolation Prizes - rarely do I let a skill challenge be only about one thing. Not only do I let the players roll whatever skill feels appropriate to them, I like to let them define 'side bets,' so to speak, for other effects that are related but tangential to the main goal. F'rinstance, your Fighter or Ranger may actually be useless in treaty negotiations. That's not uncommon. If you want to make Endurance and Perception checks to stand guard and not be surprised in case a breakdown in negotiations leads to a sudden attack on the fort, be my guest. Conversely, maybe you think it's going to go well and you want to use Streetwise to ensure your group's proper fame among the local populace when it's over. Have players who can't directly contribute well set up "if we succeed, then" or "if we fail, then" contingencies
2) Active Opposition - I'm really keen on Skill Challenges that 'bite back' and force rolls from PCs that they may or may not have wanted to make. This is counter to your 'agency' theory, right? Well, it would be except that you now open up a second branch of effects for your off-skill players to deal with. Sometimes, you need dedicated damage control. So maybe your wizard is absolutely worthless at tracking the horrible beast through the forest. He CAN however use Arcana checks and Prestidigitation or maybe the Banish Vermin ritual to ward off the insects that are draining your group's Healing Surges. Your Warlock probably isn't fast enough to chase that gnome rogue who just stole the MacGuffin but he can Intimidate the crowds of people that the rogue runs through to make you roll extra, high-DC Perception or Acrobatics checks. Skill challenges currently assume that success or failure is entirely in the PC's court. Throwing a couple extra disadvantages at them can keep the second-stringers on their toes and involved.