Uploaded a diff (using `arc diff`) and set paths for `arc:generated` with `arc call-conduit 'differential.setdiffproperty'`. Loaded the differential in the UI and noticed that the paths were marked as generated as expected.
One interesting thing to note (I'm not sure if I would consider this to be a bug or expected behavior) is that lint messages takes a higher precedence over generated paths. If a path matches a regular expression in `arc:generated` but has some linter messages associated with that path, then the file will //not// be shown as generated in the UI.