Ask HN: How to get entire list of all registered .io domain names?

I doing some research on the trends of .io domain names. I'm having trouble getting a reliable list of all registered .io domain names, though. I've purchased a few curated lists, but they were obviously incomplete. The .io registry does not supply access to zone files like verisign does (use to?).

Anyone know of a reliable way to gather the entire dataset of registered .io domain names?