The Russian Dragon* did precisely that back in the '80s, and I believe the Beat Bug also monitors tempo. LiveBPM is available on Apple & Android devices, and it even graphs the tempo variations. It has become the preferred tempo monitor for most drummers, and is sufficiently cheap at US$3.99. Also, I can tell when I'm with the click, I don't think I'd benefit from the visual anxiety of the inevitable few milliseconds difference.
However, the Accunome would make a good app - at least its core elements - as a cost-effective alternative to LiveBPM. That is, only charge $1.99.
Bermuda
* Rushin Draggin', get it?