You can create a new map that only has unit images, and depends on another map.
Say you wanted a revised mod with mexican units. You create a map called revised_mexican, in a folder called revised_mexican, with a units folder, a games folder containing your new game, and a file called dependecies.txt. dependencies.txt has one line, which says,
dependencies=revised
When the game looks things up, it will first look in the revised_mexican folder, then the revised folder, and finally in the images folder (all maps have a hidden dependency on images).
A map can depend on multiple maps (pulling in units from nwo and the map from revised say).
I assume that the xml file would need to reference the map as revised_mexican.
Could this technique be used to point to a folder other than the actual map? For example, lets say I wanted to point to the units from 1 scenario and the map from another. Could that been done.
yes, the map name is revised_mexican.
You can depend on multiple maps, for example, to depend on revised and classic, just do
dependencies=revised,classic
Then when looking for resources it will look first in the revised_mexican folder, then revised, then classic.
You can't depend on part of a map though. If you want the map from classic, and the units from nwo, you are out of luck.