That Garmin cable is way too big and bulky. You can get a better cable, or just a connector, from
Pfranc.
I bought the connectors, and made my own cable for use with my Garmin in my car, connected to my Palm running Mapopolis mapping software. With that I can get voice directions to any address. There are also European maps.
For use in this situation, a very thin cable with 3 conductors a couple of inches long is all you need, and is easily made. I would be worried about the connection on the outside of the bag, though - how do you keep water out of that? A little salt water, and your data is shorted out.