Heya!

For PS3:
I guess they just have to implement a combo-key that lets you activate the TextBox, it'll be showing the digit bar and then the Keyboard Menu will be prompted like every PS3 game.

For PC:
I guess they just have to implement a combo-key that lets you activate the TextBox, then it'll be showing the digit bar so you can use your mini-keyboard embedded with the controller.

In conclusion, the solution here is simply a combination of buttons that will activate the chat.

See ya!