Peel and Eat Shrimp
Are there any techniques for improving the peelability of the shell off the cooked shrimp (cooked completely unpeeled, head on) other than not overcooking?
No, there is nothing I can think of or have ever added to the well flavored poaching liquid that will make shrimp easier to peel, but, to be honest, I don't think of shrimp as that difficult to peel anyway, raw or cooked. Cooking with shell on has an advantage of retaining more flavor than peeled, and the cooked shrimp tends to shrink slightly from the shell when chilled, allowing the shell to slide off more easily.
Peeling while eating is just adds to the fun.