Thanks, you set me on the right track 😉
Formerly, the MXD behind the feature service defined the feature layer's symbology, including transparency (set to 20% on the Display tab of the Layer Properties dialog in ArcMap), though that part seemed to be ignored, so I set Opacity="0.8" in the esri:FeatureLayer element in the Map's XAML. I'm not symbolizing on any attribute, nor was I applying any renderer in XAML or code.
Then when I selected a polygon with the EditVertices command, the editable polygon changed from transparent to solid fill.
This is the 2.2 API.
Now, I assign a SimpleRenderer with a single SimpleFillSymbol (setting the Fill and BorderBrush to #oorrggbb color values that include opacity) to the FeatureLayer in XAML.
The polygons now draw with that renderer, and stay the same when they are selected with the EditVertices command.
Thanks!