Go on! You're my hero!
Riding my bike is pretty much the only place that I DON'T practice harmonica - but that doesn't mean that you shouldn't go for it.
I think that the neck-holder will be tough. It's really not made for a bent-over biking posture. And it will limit your head mobility.
I say carry the harp in your back pocket for one handed celebretory riffs on your decents!
-jf