I'm revisiting this request because I believe Tom looks to have already written the underlying code for it ..specifically with the Custom Win Condition, "Unit Count."

The feature would extend the "Unit Count" feature to factories (not just the win condition).

It is much more than a time-saver for designers.  Even in cases where a single "unit count" factory only seems to be replacing 4 or 5 factories, the bonus values are completely controllable, whereas "overlapping" factories under the current system imposes very restricted bonus structures.

Here is the Wiki with a number of examples and suggestions of how to implement.