TextFaster is a keyboard with big keys, six times larger than the regular iPhone keys. You can call it with a url scheme.  You can pass in the text to edit, and the url scheme to return to.  Calling it starts up the application, the text can be edited, and hitting the return button, sends the text back to the calling application. 

Application homepage: http://TextFaster.com
iTunes link: http://itunes.apple.com/us/app/textfaster/id338596754
HTML Link example:
Native application URL string example: keyboard://?text=Hello%20World&fromapp=mailto
Extended example:
Tags: keyboard big keys texting typing

I am happy to make changes to this scheme to meet your needs.  I just found out about the x-callback-url scheme.  Next release will support it.  I can also give you a user interface with multiple fields, for example for a text message, or email message. Send me an email.  let me know what you need.