You have to simulate the switch press down / up. Once you're in SOS mode, the On/Off, Favorite, and Tap switches are used for controlling the SOS loop functions.
CC 82 for the Tap switch: 127 for press, 0 for release.
CC 81 for the Favorite switch: 127 for press, 0 for release.
CC 80 for the On/Off switch: 127 for press, 0 for release.
I think you'll need to use a CC momentary message so the switch executes 127 on press, 0 on release. It's definitely working for me on a PBC 6X and a Volante.