As I understand it your making a repeater for the info thats contained in the watch. Interesting, I have made similar displaying flight info in biz jets for passengers, hardwired though.
I use a speedpuck on the front of my board and tom tom multisport on the wrist. I find the only instant info I want is speed, everything else, like for average speed or distance travelled the wrist mount is fine.
Your challenges will be making the device robust and waterproof but it definitely has potential especially with touch screens being so cheap now, just got to make all the boxes talk to each other

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cheers
DK