Hi! I hope I understand your question correctly, but I think I'm actually doing the same thing in my image captioning tutorial to manually correct the captions suggested by the model. Prodigy's input and output format is identical, so you can pre-populate the field you're storing the free-form text in (configurable by the field_id), and the pre-populated text will show up in the UI.
In the example recipe, I've used the field ID caption for the editable caption, and added another key orig_caption to preserve the original caption text. After annotation, you can then compare the caption and orig_caption to see if and how the text was edited. See here for the recipe code: