Interesting. When I was doing more client work I wanted to do something like that, but was never able to suss out exactly how to find such a middleman. I was a lot younger and (I'd like to think) less savvy then, though.
It took about a year to build up a good hourly rate reliably... this was done mostly just by being fast, available, and good and always keeping an eye on the next project.
At this point, I know several people who are able to make money off me, and that has always been my goal as a freelancer, more than anything. The agencies I freelance with all recognize that I'm as reliable as an employee but far cheaper as far as total cost, so they have a better margin on work they do with me.
And even though I'm charging quite a bit, since they are charging a markup on my time... well, it's just a lot nicer for everyone. If I raise my rates, they make more money (within reason, of course).