[can't find anything on this in the issues tracker, nor the 'net]

Project A ("ztest1") is a multi-configuration job that produces artifacts for Ubuntu and Mac OS X (axes values = macosx64, ubuntuLynx32), for two fake branches (axes values = x, y). Project B ("ztest2") wants access to the appropriate artifacts from A, given the OS/branch combination.

Using "ztest1/label=macosx64,branch=x" as the "Project name" for copy artifacts works, for all B combinations, but this obviously copies the artifacts for macosx64/x into all B combinations.

Using "ztest1/label=$label,branch=x" instead produces the following error.
{{Unable to find project for artifact copy: ztest1/label=macosx64,branch=x
This may be due to incorrect project name or permission settings; see help for project name in job configuration.
Build step 'Copy artifacts from another project' marked build as failure}}

The actual project name, including the parameters, is correct, but it fails. The same happens if you use "...,branch=$branch" also.

The copy artifacts help instructs
To copy artifacts from one matrix project to another, use a parameter to select the matching configuration in the source project. Example: OtherMatrixJob/jdk=$jdk
so I believe that the above syntax is correct.

The Hudson log for failed B build shows only
{{Jul 12, 2011 12:16:43 PM hudson.model.Run run
INFO: ztest2 #12 main build action completed: FAILURE
Jul 12, 2011 12:16:42 PM hudson.model.Run run
INFO: ztest2 » x,macosx64 #12 main build action completed: FAILURE
Jul 12, 2011 12:16:33 PM hudson.model.Run run
INFO: ztest2 #11 main build action completed: FAILURE
Jul 12, 2011 12:16:32 PM hudson.model.Run run
INFO: ztest2 » x,macosx64 #11 main build action completed: FAILURE
Jul 12, 2011 12:13:06 PM hudson.model.Run run
INFO: ztest2 #10 main build action completed: FAILURE}}