I have done this, but don't have any code that's easy to get to. It would be in an old version of my app that's archived.
Here's the general idea:
* You have an edit box that's off screen.
* You create what looks like an edit box with sprites and text i.e. a white sprite for the background, and text that sits on top and is centred.
* When you tap on the fake edit box you set focus of the real edit box and when you type in the hidden edit box you update the text that's displayed in the fake edit box which is centred.
This loses some functionality but the edit boxes have very little functionality anyway. For example you lose a blinking cursor line and you can't move the blinking cursor but that's probably it.
What I did was clear the text from the real edit box which cleared the fake one whenever the user tapped on the fake edit box. If you don't want to clear it every time it's tapped then add a small button to the right and inside of the fake edit box that when pressed will clear the text.
OryUI has fancier looking textfields but it doesn't let you centre the text
OryUI - A WIP AGK2 UI Framework