They already do that. There just isn't any industry that can singlehandedly soak up all the excess power (subject to constraints of proximity to power plants, distance from cities, etc). There's a chance electric cars will change this.
Paper mills are pretty good at providing large amounts of instantaneous interruptible load. I'm sure there are others, but you're probably right, soaking up ALL the excess baseload in a baseload only grid i would be a challenge.
Energy storage, using electric car batteries as grid smoothing utilities is an interesting approach. Grid sized flow batteries[1] are another interesting option.
