Garmin GNS 430/530 crossfeed

Hello,
I am looking a method to upload user waypoins to a GNS 430 deirectly
from a PC or, as alternate method, through a Gpsmap 296.

I see an official upload is not documented, while the Crossfeed
functios is used to transfer waypoints between two similar units. I
wonder if someone cracked the protocol, so I can write my own 430
simulator and use it to "crossfeed" my waypoints to the real one.

An alternate may be to upload waypoints to an handheld 296 and the
crossfeed between it and the 430. It looks someone did the trick, but I
cannot find any good refernce on the connection and settings.

Any idea?

Thanks.

Giuliano