Doesn't work. Clicking the button makes it flash yellow. But it doesn't stay yellow to indicate it's active. Clicking to place a blueprint returns the error that you can't build on deep water. Doesn't work with placing single rails either.

I have been having a slight Factorio break, but I just tried it in the current version and it works just fine. Maybe this is a usage issue? You have to keep the blueprint in the cursor and then click the landfill button. After that you should have landfill tiles in the current blueprint, and you need to shift-click to get your bots to place the tiles (and after that, shift-click again to place the blueprint. This is a Factorio limitation and nothing to do with the mod).