I need (i.e. a customer requirement) to provide a custom keyboard for the user to type text into both text fields and areas. I already have something that does the keyboard
It depends how custom you want it. I personally wanted a numeric only keyboard, but didn't like the number pad, so I used the standard keyboard and added in a standard filter, which is part of the options, which rejects all the key presses that aren't numbers or space or delete.
I can't say whether Apple would like this though, and you're going to have a very hard time writing your own behavior that acts like the other keyboards. So much so, that it should be declared a bad idea.
update based on your comment it sounds more like you just need to create a view with a lot of buttons on it, and move this view around with the animate option turned on. It could then sort of slide up from the bottom like a keyboard and slide away again when dismissed.