IP address is pretty definitively regulated as PII under GDPR rules. Our lawyers consider that any sort of hash or other derivative would still be PII, and thus require consent.
(While it's theoretically possible to do analytics on someone without an IP address, browser fingerprinting alone has a half life of ~24 hours. It's not a reliable enough indicator for any business purposes.)
But it's all kind of moot anyway. It hasn't been extensively tested by case law, so no corporate lawyer is going to tell you to go with the innovative black box solution.
In our industry, we all await for the completion of the ePrivacy directive. But I suspect regulators have put themselves into a bit of a corner. At the end of the day, TCP/IP is inherently a non-anonymous protocol. Your identifiers are baked into the architecture of the internet as fundamentally as your home address is known to the mail system and your license plate is to the transit system.