I would like to bring your attention to additional rules for British Cycling Technical Regulation 3.2.4 ref cycles used for track racing.

Technical Regulation 3.2.4 additional rules for 2014

Riders may carry no object on them or on their bicycles that could drop onto the track. They may not bear or use on the track any music player or radio communication system.

In addition: Any electronic device with display ( for instance speedometer or powermeter ) must be hidden so that it cannot be read by the riders.

Gordon Sherratt