I finally planted the crevice trough that I cast early this spring. It's filled with a 50% gravel, 25% grit and 25% topsoil mix. I planted it with a mix of sunny, free drainage-loving plants like lewisias, verbascum, dianthus, arabis, orostachys, and pulsatilla.
The small lewisia at the bottom of the crevice will have a chance for a nice, cool root-run while keeping its collar dry.
The top of the trough is dressed with a thick layer of grit to slow evaporation, stop rain splash, and keep plant crowns dry.
Fingers crossed that it's happy down there in the crevice.