How to build surge brakes on bike trailer? There are a couple of pretty good answers in How to make a cargo trailer?.  After looking at those, at the answer to What are the safety concerns of a high vs a low mount trailer hitch? and doing some other research, it seems that heavy trailers have significant safety issues around braking.  

All the references I found so far indicate solutions that involve disconnecting the bike's rear brake and connecting the cable to trailer brakes.  This solution has issues of its own (mainly around not going to the trouble).  

There is common solution often used on rental automotive trailers, called a surge brake.



When the tow vehicle slows down, the trailer pushes against the vehicle and that force applies the trailer brakes. In automotive solutions this is done with hydraulics.

After thinking about this a while, and noticing there are many inexpensive used kids bikes with manually applied brakes integrated in the rear wheel.

Are there any proven approaches to using surge braking on bike trailers, either with used bike parts or with new purpose constructed parts?
The simple answer to your question is yes, people have built surge brakes and they do work. Specifically, I have built one, but I called it an inertia brake.

This page has some notes and photos. The mechanism was fairly simple and robust, a telescoping towing arm with a couple of standard brake cables leading to V brakes on the front wheels of the trailer.

  

The major caveat in my opinion is that by the time the trailer is heavy enough to need these brakes it's too heavy to be towed safely behind a two wheeled bicycle. I say 'two wheeled' because in many countries legally speaking a bicycle has "two or more wheels" - so to a lawyer my quad bike is a bicycle. I built that trailer for the quad and would not suggest towing it behind a two wheeler. Mostly because if you fall off the bicycle the trailer brakes will not be applied so the trailer will roll straight over the top of you.

You may also find "does anyone sell bicycle trailers with brakes?" useful