I am a genius.
A Label or RichTextLabel can be duplicated and placed above the first Label/RichTextLabel on the tree.
It can then be modulated to any color, assuming the original font color is #ffffff.
Then, assuming a compatible monospaced font, a script can copy the text of the first Label to the background Label, and replace every character, excluding newline, with █(Unicode: U+2588).
The default Godot font, besides not being monospaced, does not support that unicode character.

This may be my hackiest work yet, and I'm unashamed.