My partial solution for this problem was installing a Flume water flow sensor on my home's water meter. While it won't alert me about an overflow, it reliably reports when I have a steady stream from the autofill. In my case, it reports something like 2 gpm while it's running and will send me a message about a "potential leak". It's not perfect for this use, as it needs a sustained trickle to trigger. It also sends the same alert for other leak events (like a leaky toilet seal) , but it's enough to trigger my investigation and as a bonus I get full house monitoring of water usage.