Frank Mir is a Black Belt in American Kenpo
I would say he is the most recognizable today.
There are several low level guys as well, but you would have to be really into MMA to know them.
I think Keith Hackney was the best example of a Kempo stylist in the MMA arena. His movements and techniques etc.
Zane Frazier was also Kempo.
I'll get back with more as I remember them