I'd say the application would depend on what you're looking for. I mean a big optical sight looking shift light I think would be utterly out of place. But if it were an LED in the right spot that it would catch your attention without being utterly distracting it could definitely be done without being an eyesore.
There's actually a pretty good write up on doing a DIY circuit for these, very inexpensively, over on RX7 club. Which is what I'd do if I were going to build a shift light.
http://www.rx7club.com/2nd-generation-specific-1986-1992-17/how-diy-rpm-switch-6th-ports-vdi-233269/I just couldn't justify tearing apart a bigger shift light just to dumb down the indicator on it when you could do this for next to nothing.
That said, does anyone know if an unlocked ECU on these has an option to pull a signal directly from it for activating an LED. A couple of the vehicles I've worked on, when installing upgraded ECUs, it was a feature.