Thank you. I did that and have now been able to set up Charge on Solar. For anyone waiting to do this, the app warns that logging out will lose your data, but my app is behaving normally after logging back in (as you would expect it to).
Setting up CoS requires that you pair your phone with your Powerwall 2, which involves toggling the power switch on the side of the battery. You can set CoS to work anywhere above 30% of vehicle battery charge (if your battery is below 30%, and solar is insufficient, it will charge from the grid). The minimum excess solar production is 1.2kW before CoS will work, which matches the UMCs lowest charging of 5A.
This function is long overdue, having been promised for 4-5 years, but I am very pleased to see it arrive. Now, does anyone know if it can work with a wall charger and a non-Tesla vehicle? If so, I'm going to need to look harder at buying a wall charger!