1) If your background is uniform, you could overlay a image with the same color as the background that has the button shape transparent in the middle
2) Using the overflow property, you can get a square mask.
3) Since -webkit-mask aren't standard (but now it looks like it might become in the future) we don't have any direct controls for it, but you could potentially add the rules yourself to the custom css part in the properties panel. This is "real masking", however it only works on webkit browsers.