It is possible to make the Ext Amp switching momentary. It's a bit tricky and I only know how to do it in HX Edit:
1. In the Command Center, assign the desired foot switch to the Ext Amp and select the type (Tip, Ring, or Both). Select the LED color and customize the label if you want to.
2. Switch the foot switch to the desired default state. This will be the state of the Ext Amp when the foot switch is not being pressed so this is important. When the LED is dim, the circuit will be open, when it is lit, the circuit will be shorted. For the Trio+ I needed it to be normally open, so I made sure that the LED was dim before proceeding to the next step.
3. Add a gain block to your preset and assign its bypass to the same foot switch and its Type to Momentary. It doesn't matter where in your signal path you add the gain block since it can be deleted when you're done.
4. Repeat for the other two Ext Amp types if desired. You can use the same gain block.
5. Delete the gain block, save the preset and you are all set.
If you did it correctly, the foot switch LED will change states when pressed and return to the default state when released, so for normally open, the LED will normally be dim and will get bright when pressed and return to dim when released.