Do you get a reliable GPS signal with the Tom Tom, or other sat navs, when placed where shown on the pics?
It should work okay placed there. But don't let placement stop you, if need be, get an external hockey puck antenna (they're small) and place that higher up/more in the clear.
Edit:
I would advice against a smart phone. They're not as good as a dedicated GPS when it comes to reception (unless the smartphone allows for an external antenna), and secondly, you have to look longer to find one you can operate with gloves. If I were you, I'd consider the Garmin Monterra (über expensive, but runs Android and you can have apps on it etc.), the Garmin Montana, one of the Garmin motorcycle GPS's (Zumo, I think they're called), or another brand of motorcycle GPS.
