BTW, I think you asked the wrong question. For example, I personally would have fed the NMEA messages into a NMEA channel on D-BUS, allowing any application to subscribe to the message flow. But to get that answer the question is about sharing messages, not about sharing serial ports.
That assumes you've got the source code to the application, and can modify it to subscribe to a D-BUS message flow. It also assumes that the application runs on an OS that supports D-BUS. Neither of those two are the case for all of the apps at this point.
The answer involved some chicanery with gpsd, gpsprobe, socat, and a Windows program called HW VSP 3 (for those apps running on Windows that expect to read NMEA data from a serial port). I'm putting the solution up on a wiki shortly -- one that's well known to the sort of people who do ocean navigation and are likely to need it.
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