Interesting the defuser dome on my SB 800 installs only one way.
You are absolutely correct... my mistake. I can lift that side of the diffuser slightly so the intact plastic between the tiny hole I made and the edge of the diffuser depresses the switch. From memory, I thought I flipped the diffuser around. That was what I initially
thought would work, but did not
.
With the diffuser pushed all the way on, the tiny hole I made allows the sensor switch to stay in the extended position:
With the diffuser lifted very slightly on the side the switch is on, the sensor switch is depressed:

I imagine if the hole was too large, a small, thin strip of plastic or card stock could be inserted under the diffuser to depress the switch...
Since I made the hole, I don't know if I've ever used it with the switch depressed as designed... I'm very happy with the way the altered set-up works for ceiling bounce, which is what I almost always use.