Advice on how to unfreeze trailer brakes

Huh? Pound on the wheels from the outside? Pound on the drum itself?
Pounding on the wheels is as non-effective as pounding on your fuel tanks. Drums are cast iron, and can break a chunk off if hit near edge & hard.
I sure hope you're joking because those are the last two things to "pound on" in order to free frozen brake shoes, and more likely to cause damage, rather than free them.

If you're not hitting the brake shoes, you're hitting the wrong thing.

 

If you're able to break a drum you have one heck of swing.

 

Is a drum cheaper than a tire? Hey, just had to ask.....

 

A little trick I learned when I sprained my wrist is ether (aka starter fluid) or a spray bottle of air line anti freeze either will melt and evaporate the ice on the shoes just don't be smoking while you do it