I created a 57×57 circular icon (without shine) which has transparency outside of the circle for my app. I can successfully install the app on Simulator and iPhone. It works fine, and looks great.
However, can I submit an icon with transparency to Apple? Will they accept it? I can't find any apps which have transparency in their icon. Instead, they use a kind of pseudo-transparency - a jet black background with no shine.
Apple may have updated the interface guidelines since this question was asked, but transparency is now expressly not allowed. See more here:
https://developer.apple.com/ios/human-interface-guidelines/icons-and-images/app-icon/