Hi there!
The demo, unfortunately, is not 100% reliable for MZ reference, as I use a sort of a "hack" to have a single demo for both versions.
However, I just run a test with MZ 1.7.0 with a pure MZ project (loaded with some of my plugins only), and I was successfuly able to change fonts registered in the core database (as indicated in the image below). So if you use the engine's font customization features you should be able to use your custom font with the plugin. Keep in mind that the default "Game Font" refers to the Main Font and Number Font files specified in this menu (if I'm not mistaken).
