Audi made a press release on Novermber 25, 2003 that announced the A8 L 6.0 quattro a W12 for sale in the European market. Orders started on December 8 and deliveries are scheduled to begin in February this year. I believe it's going to be available here in America as a 2005 model. The point I was trying to make above when mentioning the A8 is that it is just now being offered with a V12 in the third generation of the car (if you count the V8 quattro as the first) after a reputation has already been built with the V8 model.
Sure they could do that, but the Sixteen was a one-off concept as was its engine. I guess you could lop four cylinders off the V16 to make it a V12 but it would still be a considerable engineering effort.

I still don't think it would be in Cadillac's best interest to immediately enter the ultra-lux segment and try to compete against the S600, 760Li and A8 L 6.0 quattro. Those are three really freakin impressive cars and Cadillac has yet to establish any sort of presence in the premium full sized rwd segment.