You are probably correct, Lled, but my sysadmin swears it cannot be a system protocol issue because cecopy can be successfully executed under a user-initiated circumstances. The problem only manifests when fired from within modelbuilder or a python script in ArcToolbox. Double-clicking the same python script from explorer executes successfully, and firing the command directly from a user-initiated command window works as expected.
There obviously is some difference between firing a python script from arctoolbox and by double-clicking from explorer, and our protocols let one instance pass but stop another. Hopefully a knowledgeable ESRI employee will be able to tell me what settings might have this effect, because I'm pretty sure our IT staff all the way up the chain will not be able to.