I wonder if that sort of measure would have far too much lag to be effective. Also, what signals do they go off of? Ask workers and there's too many, ask industry and there'll never be enough.
I think there are some labor board requirements to report layoffs? Maybe that could be used. Could implement some rule where you can't do layoffs at all while employing visa holders. No way that an industry going through massive layoffs needs to bring in people on visas while unemployed citizens can't get hired.