I can modify ampr-ripd (and amprd if needed) to periodically sent some data "home", locator and gateway callsign to 44.182.21.1.

This would need an additional parameter, something like "-Lyo2loj at kn05or http://hamradio.ucsd.edu/mailman/listinfo/44net".

This would allow me to put together a simple tracking database and show active nodes on a world map for everybody to gaze at :-)

Yes, it could send some fields like the software version and maybe the number of routes it has put in the table, so it will be possible to debug things and be able to notify users when a serious problem is discovered or protocol change is made that requires update beyond some version number.

That could be done using MNDP or similar protocol sending to 44.0.0.1? I would recommend having different fields for different information. (sysop, locator, etc)

Way I the past I had something like this in my NET version. It used multicast to 224.0.1.31 which is a multicast address I registered for this purpose.

host 224.0.1.31 31.1.0.224.in-addr.arpa domain name pointer ampr-info.mcast.net.

You are welcome to use that address when required (probably not, just send over tunnel to some real destination or 255.255.255.255)

Rob