It's likely that ToySerialController doesn't "register" its parameters. Registering parameters does two things:
1) The parameters will save when the plugin is saved. For session plugins, this happens when saving a session plugin preset. For other plugins, their parameter save also with the scene and atom presets.
2) The parameters become exposed to the VAM trigger system and to other plugins. (But session plugins aren't available via the trigger system since the session plugin manager atom is unselectable in the trigger menu.)
Session Plugins Hook can only see parameters that are exposed via registering. You can easily test if a parameter of a session plugin is registered by setting it to a non default value in the plugin UI, saving a new session plugins preset, and loading that preset. If after loading the value is still what you set it to, it should also be visible to Session Plugins Hook.