I tried this while I was working as the recruiting manager at a large AE firm and it wasn't really what I'd hoped it would be.  It wasn't a flop, but it didn't really get us what we wanted either.  I used a firm that was very well versed in engineering search work.  What it amounted to was the hires the firm made cost about the same as the other search firms we were using and the productivity was about the same too.  My business parter was also involoved in a simular situation, but the firm they used didn't have A/E specific experience and they made very few hires over the course of a year.  They ended up being much more expensive than the other contingent and search firms he was using.  I think it could be successful but would have to have the right outside firm to run it, internally sell it to the right people in the firm, and roll it out methodically to the most receptive group of internal clients and constantly monitor for success.