Remove silks from corn but leave husks intact. Put corn in a large bowl with 1/4 cup salt and water to cover. Soak about 1 hour.
Heat a grill to medium-high (350° to 450°). Meanwhile, whirl queso fresco, butter, mayonnaise, and lime zest in a food processor until fairly smooth. Add cilantro and pulse to blend. Spread half the butter mixture on a plate and spoon the rest into a small bowl.
Grill corn, covered, turning occasionally, until tender (peel back husks to check), 10 to 15 minutes. Peel back husks from cobs and tie with a strip of husk. Roll cobs in butter mixture on plate to coat (use a flexible spatula to help), then set on a platter. Serve with remaining butter and more salt to add to taste.
*Find queso fresco at well-stocked grocery stores and Latino markets.
Note: Nutritional analysis is per serving, using half of butter mixture.