It would be up to the individual plug-in to provide a means of mapping the A/B switch to a specific note or MIDI CC value. There’s no built-in feature for this.
You can’t load different patches by way of a MIDI message or key switch. Use a multi-timbral plug-in that provides multiple channels, each of which that can accommodate a different sound, and then in your expression map you can switch from one channel to another using switches.