We always label our source that was used to build the assets as well as label the resulting assets.
I am able to label the source using the Perforce Label post build step, but I am not able to label the published assets. The reason is that the published assets are within a different workspace with a view spec specific to published assets and not source. Using the Perforce Publish Assets post build step I can submit the published assets using a specific workspace.

I think a great addition would be to label the published assets. This could be an option for the publish asset post build step or an expansion of the label post build step to provide a workspace.