(Drupal 6)

What won't(?) work: custom form element, drupal_render() and #ahah

#ahah does not look like it will work in general, because it looks like it attaches and selects things based on #id, which isn't(?) unique across the whole page if you are printing out more than one thing which you want to be editable in page.

Pure jQuery solution

Step 1: Print out a placeholder element that can be selected (i.e. has an #id)

Step 2: Attach a handler to it

Step 3: Create a drupal menu callback

Create a menu callback called mymodule/js/form

Do some craziness with the form_get_cache and form_set_cache

Return the HTML to be added with:

drupal_json(array('status' => TRUE, 'data' => $output));

Step 4: Put the result of the callback into the page