It's an interesting dilemma.
Perhaps a feature or two could make it work how you want.
The other option, and maybe what you're doing right now already, is to just have the backup audio already there 24/7 on your backup input/s. That's generally what's being done when using BreakawayOne somewhere on the STL path. I have one station that actually has 5 different ways that audio can get to the transmitter, all handled automatically by BreakawayOne. That station is NOT going off air hehehe.