I was using the liquid unloaders of Railcraft to stock multiple tanks with lava, but the buffers on the unloaders are pretty big. Therefore, it's quite hard to keep large amount of tanks/other devices stocked with lava or another liquid, which is why I request an option on the liquid (un)loader to give off a pulse when a certain amount of mB is present in the adjacent device, much like the stock function in item (un)loaders.

If I was to implement this...how would the GUI look?

Wouldn't it be something like the item (un)loaders as he speaks off.

^^ if above is not on the item (un)loader implemented I also would not know how the new GUI would look like.