Any word from iContact on this?
I don't think we need to 'enable double-optin' as much as trigger it. I have to image the API will trigger double-optin as it's a major part of email marketing.
The trouble here is - as I've spoken with them - is that they tie their autoresponder service to a verified optin. Without double optin there's no way to trip their autoresponder - from they told me. If you can find a work-around, great.
Also, you could confirm my earlier post with them about this hidden field turning on double-optin:
<input type="hidden" name="doubleopt" value="1">
If it does, can't you simply add that into the plugin on your end and see if the API captures it and passes it through to the iContact account?
If that's the case, then couldn't you just write the plugin to toggle whether the hidden field publishes to the form the plugin creates or not? As long as the API will pass the hidden field and trip the double-optin process then it doesn't really matter, it would seem, how the hidden field gets included.
Just a thought.