Isn't this already possible? There are plenty of day laborers who wait in the morning at "The Corner", and people with jobs for the day come pick them up. They work when they are able, and when there is a job for them. Regardless of whether or not this is legal, the market for this already exists, and it frankly isn't that great for employees. The benefit of a steady job -- or more accurately, a steady stream of income -- is that you can plan around it.

Also, I fail to see how regulation has prevented this from happening. Do you have any evidence you could point to? Lots of businesses have a big payroll of part-timers who only work a few hours a week. Speaking from personal experience, when I used to work these kinds of jobs many of my coworkers (myself included) had extremely flexible "work when you can" schedules. Lots of transient employees, too. For some industries this works, but for most non-minimum wage jobs frankly I don't see what the incentive for an employer to do this would be.