'Overwriting' background blocks

I'd like to be able to 'overwrite' background blocks: that is, place a different type of background block without first removing the original. Whether or not the original disappears or 'drops' doesn't matter to me. I like to make pseudo-realistic bridges, and I prefer the 'legs' to be in the background. Being able to do this without first destroying the existing background blocks makes it much less tedious. More importantly though, destroying bg blocks in a pond causes it to drain. I guess this could be abused in multiplayer situations, so maybe it could only be possible when the background block is 'natural' (unrefined dirt, stone, etc.).

Thanks for your consideration.

 

you can drain liquids near the surface of the planet
but not underground

 

For the record, I meant my suggestion could possibly be abused, not the ability to drain liquids.