I always thought this could use a bit of high tech. It should probably involve a web site to request workers and an SMS solution for people out and about. It should also have some sort of vetting and recommendation system to replace the quick size up that happens in person.
At the very least, why not have a day laborer office where people can sit in a chair instead of on a street corner. It could also double as a financial or immigration advisory role. That would be a great non-profit to make.
However, keep in mind that day-workers and agricultural laborers aren't necessarily illegal. Where I was working, most of the people working in agricultural labor were migrant workers, but were also actually legal U.S. citizens from border towns in Texas.
'Internet hackers organize illegals to take american jobs.'
Here in Phoenix, they do. Google Sherriff Joe Arpaio if you're not familiar with him.