The current strategy of COPYARTIFACT_BUILD_NUMBER_MYPROJNAME does not work when copying artifacts from two or more builds of the same project. The build number is overwritten by each artifact copy step.
We're running tests on one project and then comparing two or more results of the same test by copying those artifacts. However it would be useful to have an environment variable with a list of all the build numbers that were accessed by the copy artifact step.
Please see the Parameterized Trigger Plugin for an example of how this could be done. What they have is an environment variable that returns a comma separated list of build numbers.