Anyone made a "hill retarder"?

Are you referring to a string of cars without an engine? Don't most engines stay where they are stopped on a hill? Having said that; your idea sounds plausible.

We have planned a grade from the mainline up to the upper level, which is again I say "planned" to have centenary.  So when we are swapping to electric power from diesel or steam to transverse this and go around the upper loop, the short area to do this on the main from a train coming down the hill might leave part of it up on the hill, unless I do a lot more cat on the main so it can get to a turnout, to allow the swap to complete. 

So if I can hold it after uncoupling the electric I can shorten the amount of cat needed on the lower level main to accomplish the swap.

I think your idea would work.  I want to do something similar on my layout because I want to be able to park a small string of cars on a grade while I do some switching.

The one thing is that you will have to stop at the exact spot for it to work.  Also will a switch machine be enough to hold a whole train of cars.   I think some simple engineering and you can come up with a really good solution.  

If I can get the switch machine to make the bell crank come down and release it with the pressure on it from the train it might work.  The one I have has a big green coil and a bell crank that throws one way or the other and stays there unit thrown back.  Maybe a spring to help it come down, but not strong enough to prevent the rod from going up.  There wouldn't be a load going up, but pressure against it once the train is on it.

I think he said that the switch machine would operate a bell crank; so it would be a mechanical brake operated by the switch machine.

I've actually been thinking of of doing something like this for car stop for a hump yard.

Why use a bell crank at all? Why not just mount the Tortoise sideways and have an L shaped wire stick up and catch the axle? The wire would have to be fairly heavy, thicker than what comes with a Tortoise, but that's an easy modification.

You are correct - a moldy-oldy idea.  One of the HO layouts that I operate on has several of these in sidinggs that have some grade to them.

One only has to remember that the "brake" is one before pulling out of the siding....

Wow, I remember reading this article too in a Model Railroad magazine.

In HO I had a 7 car siding that was on a slight grade so I glued some tall grass between the rail that made contact with the axles. It was just enough friction to hold the cars.

For O scale mass is the issue, the question is how many cars and at what grade? You could look at gluing a series of broom straw cut from an old broom to hold the cars. You may need quite a few.

They are very resilient from breaking and may provide enough friction to hold the cars.