How can you prevent the lettuce from wilting if covered with something hot?
Wilting in greens is triggered by temperature, pH, and salt content.  To reduce wilting, you can cool the vegetables or shrimp, make the vegetables more acidic*, or decrease their salt content.  

My suggestion would be to cool the shrimp with an ice water bath or cold running water.  This is the most traditional approach for shrimp salad.  Alternately, you could apply an acidic dressing before topping, but wait to salt it until the shrimp have cooled to 140F. 

Alternately, wilting of vegetables adds a rather interesting variation in flavor or texture to a salad.  This technique has been desirable and in vogue at various times.

Why does this all happen?

Plant cells wilt when cooking breaks apart the cellulose-based cell walls, and allows water to escape, causing them to soften.  To quote On Food and Cooking (pp 282): 


  When the tissue reaches 140F/60C, the cell membranes are damaged, the
  cells lose water and deflate, and the tissue as a whole goes from firm
  and crisp to limp and flabby.


Acids will impede wilting, because the cell walls are held together with hemicellulose, and it is "not very soluble in an acid environment" (pp 282). Table salt is a problem, because "its sodium ions displace the calcium ions that cross-link and anchor the cement molecules in the fruits and vegetable walls, thus breaking the cross-links and helping dissolve the hemicelluloses" (pp 283).  Calcium has the opposite affect, so if you can use hard water or add calcium salts, do so. 

*Acidic solutions will reduce green colors(On Food and Cooking, pp 280-281), but preserve texture (282).