CopyArtifact works great for it's primary function. However, if you're trying to work toward avoiding actually using jenkins artifact copying/transfer code (due to it being slow or wanting to avoid data IO from the master) - it isn't directly useful.
What I'd be very interested in would be some way to leverage the plugin for it's upstream project detection/file analysis/etc - but not actually do the copying.
Here's two things that I believe would be easy improvements that would allow it to be used this way:
1. Add a checkbox to the build step: "Detect only, do not copy file."
2. Add an additional environment variable COPYARTIFACT_BUILD_PROJECT_%s functioning equivalently to the COPYARTIFACT_BUILD_NUMBER_%s except providing the name of the upstream project that was pulled from.
The combination of these two would allow using all of CopyArtifact's detection/etc. functions, but I could instead hook in a shell build step and use the environment variables to more efficiently transfer the files, including options like leveraging a local cache/etc.
I think it would be great to go beyond this and have a plugin that could get info about the upstream build, contents, etc. and make it available to the current build, but that's a bit too off-topic I think for this plugin.