Why do you use mappoint to store a tracklog? I'm sure there are other programs who can do that, or just build your own program, it isn't very difficult if you know the NMEA syntax.

But I'm not sure at wich interval the GPS receiver sends its position using the NMEA interface. Maybe every 2 seconds? Then you're out of luck. Maybe the Garmin interface sends the position more often, but MapPoint is only compatible with the NMEA interface (stupid, but just write your own COM add-in which reads the Garmin signals - lots of work, maybe).