The QMD Stroke Trainer (www.cue-md.com) shows you where you hit the cue ball along with a lot of other measurements about your stroke. But all the electronics you need to figure this stuff out (power source/regulation, radio transmitter, accel sensors, gyro sensors, mcu, etc.) would be tough to pack into a cue ball.
11mA lithium ion battery, no regulation, simple FSK transmitter (and receiver), appropriate sensors, extremely-low-power mcu, and optimized sleep states. Shooting for at least 4 hours of use per charge. Hardest part might be charging mechanism (wireless, photo, contact?) All on a super-tiny board.
I've used the Q-MD, its clunky and asks a lot from the player to use properly.