I just stumbled on this thread in Endless Sphere, which has piles of links to other threads as well: “2-speed dog transmission”
Here’s a little video showing how it works:
It would need to be beefed up, but it’s a really standard, proven design. Here’s a link to the drawings.
I just checked back and saw his progress. It’s awesome. If you’re squidgy about watching fingers close to spinning gears, you may want to pass on this video:
His build thread is here, showing his final transmission mounted on a bike: Thud’s first build
Awesome stuff, and apparently the drawings were first made with a Motenergy in mind. (!!)