How does one set Dwarf Fortress to run as a fullscreen window?

You can adjust the window size via the init.txt file (which is in the data/init subdirectory of your Dwarf Fortress install).

You probably want to find the WINDOWEDX and WINDOWEDY keys and adjust them:

[WINDOWEDX:2560]
[WINDOWEDY:1440]

(This may need tweaking, since I'm not sure if DF uses these numbers for the size of the window or the size of the window's client area; you want the latter).

I don't think there is support in the game for windowed fullscreen otherwise, so hopefully your script does still correctly adjust the window style to eliminate the borders.

All you gotta do is change [WINDOWED] to NO