Cool idea for a project I think but commercially viable? I'd say maybe not...

This is the problem I see... double-press a button or miss a press and the system will be out-of-sync and get shut off because at speed you don't have time to divert concentration to fixing your unit.

WRC co-drivers can adapt to wrong notes, missed turns, minor accidents. An computer can't (easily, that is) What would be cool for DH racers is a video/audio link using a helmet cam. A coach could then give pace notes from a course location of his choice.