I found in the d/rails s logs: No such file or directory @ rb_sysopen - /src/app/assets/stylesheets/plugin-cf.scss.
So even though the plugin is symlinked into the /plugins folder as county-fence, and my plugin is explicitly named county-fence in plugins.rb, some code is reading the underlying directory name plugin-cf and making assumptions about what the compiled CSS name for my plugin is.
Is this a bug or a feature? 
I think the moral of this story is do not under any circumstances name your plugin directory anything except the matching name of your plugin. A symlink named correctly is not sufficient.
EDIT: also register_asset "stylesheets/common/index.scss", plugin: "county-fence" is a workaround that forces the plugin name to be set correctly.