Sprout Forms: Map Element ID to Content Field on Save Entry Event
You're almost there, as you can see on the SproutForms_EntriesService Class at line 197 we add a SproutForms_OnSaveEntryEvent. When this event is fired you can get the form entry submitted within the param 'entry'. All you need to do is add a listener to catch this event, please go ahead and add the following code to the init() function of the plugin of your custom field type:

craft()-&gt;on('sproutForms.onSaveEntry', function (Event $event)
    {
        $entry = $event-&gt;params['entry'];

        // Let's assume that your field handle is "custom"
        $entry-&gt;setContentFromPost(array(
            'custom' =&gt; $entry-&gt;id,
            )
        );
        // save it again. (avoid an infinite loop using sproutForms()-&gt;entries-&gt;saveEntry)
        craft()-&gt;elements-&gt;saveElement($entry);
    });