Let me describe to you the what I'd like to see out this hypothetical agent, named "Sam":
* Sam knows me and my personality, not just my resume bullet points.
* Sam of course knows the technology more than just the buzzwords and has at least dabbled in writing code in the domain for which he recruits.
* Sam has a sensitivity to the fact that I would like to progress in my career. I don't want a gig for which I'm "well-qualified"; I want a gig for which I am barely qualified, and can grow+learn in.
* Sam will keep tabs on me even when he doesn't have anything for me, and I don't want to move, just to learn what I'm working on, what I'm excited about or would like to work on, and how I feel about my career at the moment.
* With the above knowledge, Sam can be my advocate and will not try to wedge me into jobs that are clearly dead-ends, either from technology or career perspective.
I readily admit that this might be too much to ask. However, both the demand and pay are quite high for good developers at the moment, so I think there is room for an ongoing relationship, and a more personal touch.
If you had this relationship with your developers, word of mouth might be a satisfactory means of getting more.
However, short of that, there might be some opportunity in promoting your developer's work (as much as is possible, at least) on a blog or somesuch, and become a brand that other developers would like to be part of.
Hope that helps.
That's why we have agents for Hollywood---the deals are so big, but they're transient. No one expects to get signed onto a TV show, then still be there 10 years later.
Also, agents help renegotiate contracts year to year.
I think there's too much in the way of recycling canidates because we can't look past their past work.