John, thanks for those comments. I'm starting to think that there is a way forward.
We have a fairly old Garmin GPSmap 162 which outputs through a RS 232 plug, which then is connected into a Keyspan USB hub. That hub connects to a small PC and also to a Pactor modem, which in turn connects to the ICOM. The PC uses the GPS position to drive OpenCPN as our chart plotter. The PC also controls the Pactor and, through it, the ICOM. The NMEA data is there on the PC, but I can't see how you can send it on to the ICOM.
I've also got a puck type GPS (BU-353) - it has a USB connector and works well with a PC. Connecting that to the radio would be ideal as we only keep it lying around as a emergency spare, so thoughts on how you do that would be great. Thanks again.
: New Caledonia, Australia.