I have a sneaky feeling you're right. The online instruction book for the AIS600 is not the same as the one Matt has. It makes no mention now of silent mode or SRM being controlled by the plotter. The Garmin NMEA2000 technical handbook does show some pgn's for SRM (pgn 129802) and command controls (pgn 126208), so it looks like it would be feasible via N2k, but they're obviously struggling with the software or certification.
On the plus side, it means we've not missed something in all the plotter menus. On the downside - there's more wiring for him to do