This is an older article but somehow I missed it (or maybe I just skimmed it earlier and forgot!)
MapPoint 2006 utilities - Network World
Anyway, it mentions GPS Manager and Franson GPSGate.
"But you can't use GPSManager and MapPoint at the same time, because they can't share the serial port the GPS is on - or can they? You betcha they can. The solution is called Franson GpsGate."