What's the right way to exclude certain files from a build, using Sencha Cmd?

I'm running "sencha app build production", not "sencha compile", but I'd like to use the "exclude" commands described here.

According to the .sencha/app/*.properties files in the project template, you can set the "build.operations" property to specify the exclude string.

So I tried something like:

sencha config -prop "build.operations=exclude -namespace=my.namespace" then app build production
But I get:

/path/to/project/.sencha/app/build-impl.xml:165: The following error occurred while executing this line:
/path/to/project/.sencha/app/build-impl.xml:158: com.sencha.exceptions.ExScript: Wrapped com.sencha.exceptions.ExArg: Unknown command: "exclude -namespace=my.namespace" (x-app-build#291)
   runAppBuild (x-app-build:291)
   [anonymous] (x-app-build:571)
   x_app_build (x-app-build:569)
   <script> (anonymous:1)
I know you can do this by setting the property directly in one of the Ant properties files. However, I want to be able to specify the exclusions list at runtime, since it changes depending on what I'm building.