Hey Mathieu,
Well, it's really hard to get a simple whois result in their new website. The only way I've been able to do it has been to do a domain search from their homepage (links below) and then click on the "Ver datos" on the registered names and answer the captcha question.
Maybe you want to update jwhois to just point to one or both of these pages now.
I'm not aware of any other APIs supported, but looks like they're going into so much trouble to make it hard to check registration status. So, I wouldn't be surprised if they actually do not provide any other solutions.
Cheers,
-b