i know this may be a old post and all but there are two ways to go about this.
#1. the Bait bike guys now have a $150 head tube kit http://pegtech.com/products/gps-bicy...ct-information
#2. you can fabricate your own DIY system. Amazon sells this entry level kit, might be simple to hack the thing up and throw a good size flexible lithium battery pack in with it. http://www.amazon.com/cost-real-trac...st+gps+devices