It should work, except that if your plants really are planted on a mound, you're likely to get water running off and not getting to the roots.

Probably a good thing where I'm at -- everything's clay! I have a soaker hose anyways and it gives me more control over how much water they get. I hear they actually get sweeter melons if you slack off on the water when they're ripening because it concentrates the sugars, where as too much will dilute them.

If you have the soaker under the plastic, that should work well. While the plants are young and vining, they do need plenty of water.