Granted, his preferred way is to redirect, but he did ask if there was a way to change the display on a per-application basis. It might not be redirection (which might be possible, for all I know), but it wouldn't take more than a second to exit Gaim, and then restart it with gaim -display x.x.x.x:0, if he's not at display y.y.y.y:0 anyway.