That one was surprisingly easy, then hard, and eventually pretty easy again.
In most GUI-frameworks, I think, one could have just used the normal text-on-canvas functions to draw some text on top of the OpenGL canvas. At least that's what I had expected.
Well, it doesn't really work that way in Racket's GUI framework. So, text rendering in OpenGL it is.
Some googling later, I was horrified - the way one renders text in OpenGL is basically the following: