Hi Bruce
Here's a shot of the "brake shoe". I ended up with a "no-rotate" washer that consists of a rubber disk and thin plastic spacer both inside a little brass pan. The first pic shows the rubber disk and you can see the pan it sits in. The washer also comes with a brass flange that I discarded. Buy it at the hardware store where the faucet washers are.

I drilled a hole in the back of the pan, just wide enough to accept the Macaps pin. The rubber and the brass pan are thick enough to enclose the pin without it protruding, so the pin can't engage, but the pin does hold the washer in place. I still needed to use a little adhesive on the back of the pan so it wouldn't tend to rotate around the pin a little when the motor engaged.
At the extreme fine end of the grinders range, this washer actually was compressed a little too much - too hard to turn the collar - so I removed the little spacer inside and it was just about right. It doesn't drift in use, and is easy enough to turn.
When I eventually opened up the grind setting, and tried to grind beans for drip, I did see some drifting, so I replaced the original spring (left) with a stronger one (right) also a common hardware store item. That lets the brake work for cowboy coffee or coarse French press if desired.

It should probably be mentioned that some might be uncomfortable with a simple spring loaded friction brake like this, preferring a more positive stop on the upper burr to provide reassurance that it couldn't rotate off if the motor caught on it somehow. For commercial use, I don't think I'd trust it, but for home use, grinding a few spoons at a time, I don't see a problem.
BTW, the guy at Espresso Restorations has a simple but more robust brake mod design. His is a little more tool intensive, involving drilling and tapping the collar wheel. See it here:
http://www.espresso-restorations.com/Macap.html