Hi manewal,
Welcome to the forum!
No, you’re not a fool at all 🙂 . I like to experiment (with the WEPS) as well! In this way you get to know it best and, ultimately, the best results.
I guess the reason people use balsa or leather or similar backings is that these materials contain small holes or crevices that hold the abrasive particles of the stropping compound somewhat in place. In addition, they have a bit of “give” to them which makes them somewhat forgiving to errors.
Glass doesn’t have these properties. But it is very flat, which is an advantage. So I’d say, give it a try! Maybe the stropping compound is wiped off after doing half a knife. Well, then you know that. But maybe it isn’t and it works great.
I’m curious. Please let us know!