Harold: The answer to Al conductors on a Cu bus would be dependent on the lugs used to terminate the conductors being suitable for Al conductors, and Cu mounting. Most are dual listed.
As to the lugs being suitable to mount on Cu, the mfg data would answer that. The lugs that are factory attached to buss (a MLO panel) are usually dual rated. The question of suitability of field installed lugs on buss (industrial) is 'show me the mfg data'.
