1x1 building with one pipe input and output. It has a slider for how much is going through every 60 seconds, hence allowing better control over mixed belt inputs.
Add two modules, one for reading throughput and one for setting the slider. As mentioned in the chicken farm module proposal, we might need a generic slider control module.
This should be for all transports, be it belts, pipes or molten channel. That should make it match all cases regardless of what the game and mods comes up with later.
It's a bit of an open question if this should be a standalone mod or part of programmable controllers.