Agree this would be nice, and I also have a similar setup where main breaker is turned off and generator back-feed for power outages (interlock). Unfortunately the Vue2 can’t tell you when the grid is back if it is isolated by the main breaker being off – no current flows through the mains sense clamps.
The way automatic transfer switches determine grid up/down is by having an uninterrupted voltage sense connection to the grid, or an electric field sense device. We don’t have either of those methods available with the Vue2 hardware and accessories. Nice idea for a future enhancement though.