You won't be damaging the TDI cluster by using its tach face plate in the GLX cluster. Its very easy to restore it back to original condition should you desire.
Using a snipped rectangle from the GLX face plate might look good, although it actually might be more noticeable because of its "relief". Seems promising, though.
The "Unleaded Fuel Only" issue could also be addressed by using the TDI fuel/temp/MFA face plate in the GLX cluster and enlarging the LCD display window. But you would have a very hard time duplicating the factory appearance of that window. All 4 edges of the window are beautifully rolled out to contact the display screen. A sheet metal shop may be able to do it. I gave up on trying to do it myself. If you're willing to settle for a non quite stock appearance, you can always forego the rolled edges.
Its a lot easier to use a rectangular graphic, but the results depend on how closely you can match the black background. When you look closely at the face plates, you'll see that they are painted flat-black. I don't know how you duplicate the pigment-flecked flat look on a graphic, but just closely matching the greyscale gives pretty decent results. I've found that different printers produce somewhat different shades from the same chroma or hex number, so specifying a color is only a starting point. A greyscale somewhere in the range of #353535 to #555555 (don't remember the exact hue) worked for me. Lug Nut had good results with #455145. You can always take the opposite tack and make it MORE noticeable with a big "TDI" on it, using the factory font.
Your idea of a complete set of overlays would work best if it was template-sprayed rather than a lasered graphic. Not only would the flat black texture be harder to duplicate, so would the luminescent Dayglo red paint of the redline/empty markings.