What I want is something discrete that will give me a specific tempo _before_ I perform a piece. Nerves distort my sense of time making me start off too slow or too fast.
My snark clip on tuner will flash a little heart-shaped light at a specific tempo; that's getting close to the idea but it's a pain to adjust and it doesn't remember it's setting.
Instead, imagine a device you put in your shoe and that transmits to your clip-on tuner where the tuner then displays the tempo you're tapping. So, if you want to play a piece at 96BPM, start tapping your foot, check the tempo on the display, adjust your foot tapping, and away you go..
"An amateur is he who takes up the study of an instrument as a relaxation from his serious occupations." -- Sor