Is there a way to change the OSX system font size per monitor? I'm wondering if there is a way to change the OSX system font size per monitor (e.g. larger for Retina and smaller on external display) ?

I know how to change it system wide (as covered by How can I change the system font size in OS X?), but I would like to do it on a per monitor basis.

Note I am not talking about changing the display's resolution - Macbook Pro Retina running at native resolution.

I understand there some challenges with windows that cross display boundaries, but my thought is the system/tool could use the default for the display that contains the greatest % of the window.
No. There is no way to change the OSX system font size per monitor per window.

The key is that each separate OSX window is rendered as one. With that, each window can span over, or moved between, multiple monitors. When a window is part of multiple screens at the same time, different font sizes at different parts of same the window would be inoperable.