I think with the market you would be aiming for you would want to make this as simple to use as possible. I'd ditch even really making much mention of CRON and make a solid and easy to use UI for setting up the tasks. Your really aiming for the crowd that uses basic shared hosting so the simpler the better with the web service taking a back seat.

As a business model possibly you could make viewing the results of jobs and stats visible on the website only for people that use the UI to create. People setting up through an API are less likely going to be able to be monetized anyway.