DIV mask implemented in JavaScript/CSS
The code looks fine to me. Just a couple minor points:


Why do you change the div's position to relative? If the function is called on a div with absolute positioning, it would mess up the layout. And you should revert the CSS changes on RemoveMask() to what they were when Mask() was called because the div's content might depend on a certain overflow value.
If the div changes size while masked, the image will no longer be horizontally centered. I would suggest using CSS instead of JS to center it:

.MaskContent {
  background: none;
  position: absolute;
  width: 32px;
  height: 32px;
  top: 30%;
  left: 50%;
  margin-left: -16px;
  z-index: 5;
}

Your mask background currently only supports webkit browsers; you should add the CSS3 variations for the other browsers.