Actually, I seem to have found a roundabout way to copy the code I want.
As previously described, I have my SKY+ device (partially derived from the SKY+  device in the IR database) and a temporary TEMP device (an exact copy of the  device). Within any of the MX-850 Editor menus, these are referred to as SKY+ and TEMP.
While in the edit dialogue box for the problem Down hard button of the SKY+ device, I selected the TEMP device and the IR code for its Down button. After saving this, I then redited the button to change its source to the SKY+ device. As the stored code was originally from the same IR database source it accepted this device change while leaving the IR code unchanged. I then deleted the TEMP device.
Things would be a great deal easier and more intuitive if the MX-850 Editor followed basic Windows conventions and allowed straightforward copying and pasting of any data!