Frank:
With all due respect, the D&RGW was a different railroad company from the RGS and there was absolutely no reason on Earth for a K-28 ever to stray over to RGS trackage.
Don't spend any energy on speculating on "optional" movive power leases than K-27's to the Southern. It's pure fantasy. By the time it ever would have been possible, there were only three 470's left and the D&RGW needed them all for switching and Branchline service.
On the other hand, that's why God gave us models, so, what the Hell? Rule 1 is always "it's MY railroad and I make the rules!" Go for it.
Mike