Had this same idea a couple weeks ago.
cheap and easy way i figured it out was
Camera = ai-ball cam with mount
Trek - Ai-Ball
Tripple tree mount = Scoshe iphone 5 reqcase ( for bicycles) Have to modify it a bit.
Battery adapter and splitter to charge the iphone and use the cam.
I think total for the gear was 110, and Ai-ball already has an app for the iphone and android.
My thought process is i dont ride in the rain , and if i get caught in the rain then i put the ball and phone in the bag in my pocket.
I researched cams and displays and actually this was the best solution i could think of.
I'm in OCONUS until May 2nd so wont get a chance to try it out till then.