Possibly "horrible". Insulation does not prevent freezing, it just slows it down. ANY water will freeze, regardless of whether it is "pressurized" or not. Depending on convection to carry heat to an enclosed pipe is NOT a sensible idea. Heat tape, in an inaccessible location, would be "non servicable" if something happened to it, such a a burned out wire. Since a handheld shower's "connection" can be anywhere, regardless of where the nozzle itself is located, why do you HAVE to put the piping there?