Custom field widget with text field formatter?
I recently needed to create a widget for file field, that among other things needed to be able to handle multiple uploads via plupload.

Anyways, during the process of getting everything to work, I discovered the #value_callback attribute that you can put on a form item. It allows you to change the data of the submitted data and extra extra infomation, like fid of a file.

This isn't documented yet, but you can take a look at it's use in the file module. There is an issue for this on the Drupal issue tracker, but no good patch has been created with some document example yet.

I'm not sure if this would be a good case for the use of #value_callback, but it seems to me, that it could be of good use in this case.