I keep waiting for some serious content creator to try a bitcoin micropayment system. From there it's a small stretch to envision a third party like this one to take care of SSO and varying pay rates per provider.
Maybe there's a hidden market waiting to be tapped, and I'll look like an idiot when BitPass™ reaches a one billion valuation, but it looks to me like you have a good reason to wait instead of doing it yourself: it's real damn risky.
Why not 'regular money' that more people have?
Actually that's not it. What I'm saying is that you can have any normal multi-party signature scheme, where every N signing-rounds you ensure some measure of canonical ordering by publishing the hash in the bitcoin blockchain. I think sidechains are probably formulated in a similar fashion but it's been a while since I've looked at exactly what they are doing, the trickier parts if I remember correctly are transfers of value on and off the original chain.