I've just started to train biking so yesterday when riding I was thinking about measuring climbing and downhill.
How about putting some kind of spiritlevelsensor on the bike that communicates with the watch? As soon as it's calibrated it should give very good readings on what angle you are biking in. Maybe barometer is very good,I don't know about that. In that case this isn't needed. The good thing with the barometer is that it works when running which his doesn't but I still think it could be very accurate.