The key is to do it in a different way. People on the Weta forums are saying Weta could do an 'Isengard' environment that includes the ring-wall, and be different enough to not seem like a re-boot of Orthanc, the same way Weta did a mini balrog or smaller 'attack' troll. As an Orthanc owner, i wouldn't mind that at all, i'd even buy it aswell.
So maybe the key to SS doing, say, a Ringwraith on Steed is to do it differently - a running pose maybe, or with the steed reared up with the wraith holding a sword aloft as they do at the ford of Bruinen when facing Arwen.
I don't see how they can do the stone trolls differently....that's an environment anyway

Or the Watcher.....unless they have it holding Frodo or ripping down the doors of Durin.