The lane blocker would block items on the chosen side of the lane, allowing items on the other lane to pass through. Which lane is blocked would be switchable.
To explain it better, here is my current setup. I am using a filter inserter to allow only coal to continue, still, a small bottleneck is created.
Another side-effect is the the coal now occupies the center of the new c-belt.
With the blocker I could just block the left side (iron), allowing coal to continue (and keep being on the same lane too).

Just putting it out there, but I'd understand if you want to keep things more LEGO-like and simple. (That goes for the "side inserter" idea in that other thread too)
