It's basically self-explanatory. I would love to be able to detect how full a large tank via redstone signal. It would allow for some fun automation trying to optimise certain processes, for example trying to keep the LSB at max temp until the tank is full, making it more coal-optimal if the player is willing to build a little circuit to control it.

I think it would make the most sense for the signal to come from the hatch, though this does make me wonder if something like a redstone hatch that could be generalised to more machines would be a better idea.

yes, please!

To anyone else who wanted this capability - you can sort of emulate it by using Create's threshold switch blocks. They work, but are a bit finicky, obviously require Create, and take up more space than a functionally equivalent redstone-outputting hatch would.