The best way of assessing someone's value in this kind of work is to work with them. Extracting that second hand, through recommendations and such, seems to be substantially less reliable but perhaps there's some way to engineer around that? I wonder what you'd get if 1) recommendations were public, and 2) they were explicitly a statement that "I have worked with this person and they are better than me at X".
Yes, that's obviously impractical. But there are people who have that knowledge, having worked with the various candidates. If you can come up with a reliable way of getting that information out of their heads and into your database it would be hugely valuable. I recognize (and mentioned) that it's not a trivial problem.
