I accidentally flooded a world with lava.  I tried using WorldModify to fix it, but looks like its Fill feature would only work if I had a way to select the Air / empty block.  Is there an equivalent to a /give command, or some other way to remove all of a single block type from a world?

What is WorldModify?

If this is singleplayer, there is no way to unflood the world aside from turning block physics off (in misc options) and loading an older copy.
 

If this is multiplayer (MCGalaxy), you can use /replaceall 10 air

This is WorldModify.

Thank you though, I was able to load the world in MCGalaxy, use the command, then move the world back to singleplayer.