I am sure this is not possible. I am afraid - if you put something above the map (even if it is transparent), this will not allow you to interact with the map - since you will click on this div, not on the map. In addition, IE seems to have some problems with disguising google maps.
You may have an overlay that disappears when you hover over it using jquery or: hover clss to move it to a negative z-index, but I really don't think you can do what you need to do.
(ps - I tried to comment on the answer above - but for some reason I couldn't - sorry for the new answer)
source share