I've routinely worn US rank in the Middle East and in Afghanistan. In Saudi immediately after 9/11 I wore what I thought was a Major's leaf - in turned out I was wearing the rank of a LCOL (ie silver rather than gold - albeit toned-down). It got to the point here to change it would have seemed odd; a quite chat with my boss and he was happy that I remained 'pinned up'. It's pretty common amongst most ranks. Of course Wg Cdr in the USAF has a different meaning from Wg Cdr in the RAF - so I routinely described my rank as LCol (anyway many of their databases for passes etc couldn't cope with non-US rank).
In AFG, wearing US rank was not only helpful to the US CoC, but to the British Army who, it seems, regularly struggle to identify RAF ranks - as one wag said - without the aid of a bar-code reader.