Have a look at Bangle.js. It's nominally an open source smartwatch, but the difference between that and a bike GPS is just a handlebar mount. v2 has just gone on Kickstarter - I've ordered one specifically for bike mounting.
Would you be interested in cooperating on that? You can ping me over email (see my profile or the blog).
My second impression about the Bangle.js is: "oh no, JavaScript only?". Still, it's much better than building the same thing from scratch, except it may still take effort on the software side.
Having another interested person involved would be amazing for my motivation! Btw, one of the reasons I wanted hackable hardware was to map road surfaces on OSM ;) GPS alone doesn't add much to OSM in my area any more.
