I'm always interested in adapt my mechanics to a wider audience.
As for true20, I've never played the system. I will say that if it has a much wide skill variance than
4e my system is not likely to work well.
What
4e gives me is a pretty reasonable assurance of where a party's skills are going to be. Not perfect of course, but the variance isn't that large. You couldn't apply the same logic to
3e for example, where the skills can very by as much as 30 in some cases.
What would likely happen in a system with wider variance is that the higher skill guys would carry the party through the challenge, and the challenge would have to be tailored to them and not to your low skill guys.
Edit: Iwatt, I also read that skill challenge you posted in the link. It actually just gave me a very interesting idea on how to have a failure system that doesn't penalize players for participating. Unfortunately I get on a plane shortly so I won't have time to throw down some numbers, but it feels like a brain storm!!