While I find the Gondor Outpost idea very creative and interesting, I something rubs me the wrong way about Rohan using Gondor units. Yes, I know the Units suggested are not actually on the Gondor faction itself, but I think a unique outpost from Rohan would be more fitting.
However, gameplay wise, it might be the best Idea I've seen so far. The only thing I think is needed to be added is a limit to Gondor units to use. Because Rohan should not have access to an entire Army from Gondor. So maybe a limit of 4 on each unit would be a better idea.