Is the Meredith effect utilized in the cowling design of air-cooled engines?
The Meredith effect is to recover radiator drag by using engine heat for thrust, just like a jet engine.  It worked very well in the P-51 Mustang, actually adding a few knots of airspeed.

It also would work well with any source of heat, including a nuclear reactor, so yes it would work with aircooled engines.

The issue is drag from the radiator.  Aircooled radials have much greater frontal drag, so it worked, just not as well.  Efforts to reduce drag in the B-36 bomber resulted in roasted engines, particularly the rearward banks of the "corncob" R-4360 radial.  Overheating is a major concern, as is drag.  Engineers try to find a good balance.