I finally got the word on DSC compatibility from Garmin.
The only garmin gps units that can receive a position and post to the map are the larger units.
Garmin says there is a hardware change that must be done to the non dsc compatible units to make it work. This hardware change is in place on larger units. I gave them some feedback regarding the complete lack of information on their website regarding DSC compatibility.
I was hoping just a firmware upgrade would eventually make it work. No dice.
The units I have tried so far are the etrex vista and a 76cs.
Here is the text of their answer:
"The GPSMap76CS will communicate with a VHF radio for the purpose of
transmitting your position during a distress call. The ability to input
another radio's position would require hardware changes within the unit that
increases its processor speed such as the larger marine units. I would be
more than happy to pass this along to the software engineers in regards to
making any updates that might make this feature available on the device. I
will also mention your suggestion about the minimal information available
about Garmin units and DSC compatibility. "
It still seems odd that it can send a position but not receive.