Some criticisms of the tool:
1) the crosshairs are too thick for the small center markers on many 8" to 12" commercial scopes and would be difficult to place
over the center of the primary center without obscuring the marker
1a) if the crosshairs are not perfectly in the center, there is no way to make a correction.
2) the illumination of the tool is one-sided so the ring is not visible in twilight without some illumination like a flashlight.
3) It does not insert fully, making it difficult to get the pupil to the focal plane (as has already been mentioned).
4) Just about the only scopes still coming with 1.25" focusers are small (75-120mm) so most of the market would have to use an adapter with your tool
I could foresee making the tool 2", though, with 3D printing.
A tool like the Astrosystems Light Pipe answers 1-3 above and also #4 since it is available in a 2" size.
One question: since the tool telescopes, how do you prevent an angular error to appear between upper and lower sections?
With the only other telescoping one I know of, the Catseye, the only way I could guarantee alignment of upper and lower sections was to use the tool fully compressed
or use a ruler to get the length the same on all sides.