I love this idea.  Would you be willing to share information on how you designed the event for blue collar workers?

I work for a non-profit community mental health agency.  We hire everything from part-time support workers up through psychiatrists and everything in between (including administrative staff). 

We've been doing in-house career fairs for the past few years and they seem to work OK for us.  We are just under 800 employees, which is large for our area.  We pick a two hour time and hold them in the late afternoon/early evening to catch those who have day jobs.  I believe we've hired from each fair up to maybe 10-15 people per event (we have on average 50 open positions at any given time).

I think what works the best is to have enough hands on deck.  We have HR and managers on site with rooms available to pull staff in for an informal interview.  Sometimes we're ready to offer following that interview, sometimes we bring them back for a second interview at a later date.