Has anyone ever fished a crankbait in heavy cover, located mainly at the bottom of the lake? 

If so, how?

You need to define heavy cover. Some consider timber heavy cover and others vegetation.  I fish cranks in both types of cover. The first thing you need to know is how deep the crank runs on the equipment you're using.  You want to bump the timber, or tick the tops of the weeds and a crank that runs too deep will not accomplish that. The second thing I do is work the crank with the rod once it gets down to the depth of the cover. One of the hardest things to do when fishing cranks is avoiding reeling them into the cover and getting hung up.  Some have a knack for it, I rely on the sensitivity of the rod to let me know when I'm about to bump a stump or get buried in the weeds. Third, carry a good lure retriever and lastly, check your line and knot often.