Well yes they do! Once me and my father were flying from DXB to LHR. I was in Coach, he was in J. He got lucky and got bumped to F. On board he asked if I could join him in F (paid of course). They said yes and asked for US$900. We refused to pay more than US$500 and so I stayed in Coach ... But yeah they do. It specially helps if you are in a premium cabin I think. The cabin crew is more willing to take the trouble for processing the transaction..