What I am doing is this:
1) An area of high sand
2) A location the size of the marine ("Marine")
3) A 2x2 location ("Marine2"
4) A big location with Low Elevation disabled over everything
5) Low dirt blended to High sand (then when your on dirt, no dust
)
Then when there is no burrowed unit at Marine, and is one at Marine2 it will unburrow and remove.
Then it creates on at Marine.
I'll tell you if it works or not after a bit of testing. If it does, I'll post the map.
ADDITION:
Ok, I made it. The zergling doesn't look much like dust, but its a start.
[attachmentid=8953]
I like my idea of making it so that you walk on dirt and it stops
I could have done the terraining a little better to make it a little nicer, but its good for now.