Couple points on this that I can think of.
Nice Idea and it would make life easier but the following problems I see.
1. Cron task is a server task and highly varies in design by each server depending on how they handle it even though the basic format is the same. Some do not even support it for customers. I doubt highly that anyone would be able to write a hack that would work effectively for that specific point. Someone could surely write a nice fancy interface like Cpanel has stick it in the admin area. However we are back to the issues of cron differences on servers. Maybe a variable section to define where the cron file is located would do it
Couple problems though. What about the people with no access to cron? What about the people who's hosts provide no cron?
2. Now this idea has nothing to do with the cron feature on a server. Basically one could write an applet that would act just like a cron and store it locally with threads to automate tasks. Nice easy way to do it probally the easiest but I am not sure about server resources on this one
It would need to be able to check and calculate the time OFTEN to be able to effectively do its duty so their is bound to be strain on the server doing it this way.