I've really got to challenge the link posted by rkukl.
That link has been floating around for at least a decade .... long before King Innovations got UL listings for their connectors. At best, it's simply obsolete.
Likewise, 3M - another company known for innovation and quality - has quite a few crimp connectors listed for use with either aluminum or copper wire. Since the barrels are themselves aluminum, and the joined wires never touch each other, it ought not matter if the wires differ from each other.
The CPSC has a pretty spotty record in matters electrical. I'd hardly look to them for expertise. Moreover, their endorsement of a proprietary product is questionable at best - notwithstanding that the method is essentially not available.
Moreover, simply using an approved device ought to allow you to change from one material to another, simply by using different screws.
