In order for the "ice coating" to do any good, one must continue running the sprinklers until temps are above freezing. It's the continuous formation of ice which releases heat.

Here's some info:
http://www.aces.edu/dept/peaches/frzactive.html

Lots more if you search using "freeze protection via sprinklers site:edu" but no quotes.

Here is a link that might be useful: freeze protection

Thanks for the information. I really need the tip for my plants.

My idea was not to use the ice as a heat source but as a protective barrier to prevent the drying/freezing of the branches with the frigid temperatures and high winds. The ice would in this model seal in the moisture. It would only be needed for about 15 hours - still I would worry that the drying winds would evaporate the coat of ice before that ,so I'm thinking that the coat would need to be relatively thick, though not thick enough to damage the trees from its weight.

There are problems in running unattended sprinklers in cold weather. A friend of mine did it in far warmer weather than this (to finish his hockey rink), and the water froze all the way up the hose, the tap and into the house, where it burst a pipe.