I think most players would have no objection with them as short-term proxies. Sort of a "look folks, for a few games I'd like to have this WoMT formation represent a formation of Easterlings." We do a lot of proxy play in our local group as people try out different armies or units within an army. A lot of times you want to be sure that it's going to be effective and fun before you drop $30 - $100 on models you may regret later
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As for a permanent unit though, I think it's really stretching it. Off the top of my head I'm just not thinking of a good, themed justification. As posted earlier it may be best to just proxy them for a little while until you decide if you want Easterlings, in which case pick up a box or two of plastics. And once they are freed up strip them and repaint them to your new standards as part of your new Minas Tirith army to squash the Evil army you're building.