One idea that just came to mind is; When shift change time comes around, stand outside the plant with a sign asking, "Do you work with illegal aliens? Would you rather work with law abiding co-workers? Stop and we'll talk about making changes."
I wouldn't be surprised if you get swamped in some industries.
I would for the following reasons:
1. Most large "plants" are unionized;
2. Most companies in the US are in the small category and much of is service related which is already dominated by immigrants. They won't talk to your surveyor;
3. How would a fellow employee know if they are "illegal"? Because they speak in Spanish? We have millions of legal immigrants who haven't bothered to learn English.
I suppose you could make a case here and there, but it would have little to zero impact on the problem. There is a reason the Immigrant Reform organizations don't concentrate on this aspect of the issue.
I speak from the perspective of owning/operating a small business in a predominately Hispanic area. I followed then current INS guidlines in documenting my hirees, but have no idea if the docs were legit, and that was years ago before the forgery business was so well-established.
However, I do think your idea of approaching sympathetic local law enforcement and guv officials has merit, especially along the border and/or negatively impacted areas. Personally, I don't think anything is going to happen until the growing citizen border patrols reach critical mass.