If it's a static IP, you can sometimes SE the information from the ISP. I've done it several times before and it's worked. I just do the 'ole "I'm in some other department, transferring a customer to you" trick. Like, say you're in customer service, transferring a customer to technical support. Then ask, "By the way, can you pull up an account by the IP address?"
With one, I had to get tech support to get the MAC address from the IP address. Then they were able to pull up the account with the MAC address. It took awhile to finally get a person to go through the trouble for me.