What you've described, wyatt, is an
auto-fill device. What I don't like about it, is that now you're introducing the nasty hard city/ground water into your wonderfully soft rainwater system.
A big issue I've run across is back pressure and back siphoning from a "cross connected" system (I'm learning the lingo!)
If the city water supply loses pressure, there can be no chance of the non-potable water from being forced by pressure, or sucked in by siphon action, into the potable water supply. I think a toilet already does this, as the valve in the tank dispenses above the water level in the tank. But we need to mix the two sources before it gets to the toilet fill valve...
...The research continues.