I'm a member of the Coast Guard Auxiliary and had an interesting
question posed today. Is there a piece of commercially available
software that will translate the lat/long data transmitted from a DSC
radio call into a data stream that can be used to plot that lat/long
on a PC running something like MapSource?

Do you mean a device that listens for DSC and then transmits that data via
serial or USB? As in, a listener for your PC that would pickup any DSC data
it hears.

Do any of the existing VHF radios that have NMEA interfaces do this?