Hi Jakub,
When we developed contour labeling for Maplex, we didn't envisage that users would want offset contour labels - there seems to be too much potential for ambiguity, especially if the contours are close together. I've certainly never seen a map with offset contour labels.
I'll make a note of it for a future enhancement though. It's too late for 10.1 unfortunately, but we can consider it next time around.
In the meantime, have you tried using halos around your labels? With a uniform background, a halo will look just as good as masking the features. If the background color is changing, you'll have to pick a complimentary color.