Very good work. I had the exact same idea right down to using the clear material.
One suggestion though, consider extending the clear portion so it extends over the scale. Then you can use a square to scribe a line so that the clear portion acts as a pointer like the ones found on table saw fences. Since the "pointer" in the casting is raised, you would have to put a spacer or washers between the scale guide/tab and the Festool stops.
Ultimately, I may fit the parallel guides with an Incra track and stops. I just bought the guides and just started using them, so I will probably finish the current projects I am on before I leap into that.
The tabs are easy though and the lack of them is really sort of an over sight on the part of Festool. Incorporating them into the product could probably be done fairly inexpensively and would definitely make the guides less clumsy.