Note that this will greatly reduce the structural integrity of the glass, in my experience. I engraved some trackpads for the heck of it, and the glass was highly prone to cracking after.
The glass on the back of an iPhone is likely considerably thicker, so that should be reduced, but I would still think twice about doing this.
Note that the zooms and pews mentioned below might work fine. Or you might want to back off a lot.