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+ [174] device in the IR database) and a temporary TEMP device (an exact copy of the [174] device). Within any of the MX-850 Editor menus, these are referred to as SKY+[174] and TEMP[174].

While in the edit dialogue box for the problem Down hard button of the SKY+[174] device, I selected the TEMP[174] device and the IR code for its Down button. After saving this, I then redited the button to change its source to the SKY+[174] 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!