I don't have a MyBB forum anymore so I can't continue development on it or my other plugins. I'll probably be back when MyBB 1.9 releases but for now I'm exploring other forum options.

I wonder if people have looked into why this isn't working?
On the confidence field, I changed it from 'numeric' to 'text' and that seems to fix the entry of floating point numbers. (It seems that 'numeric' became 'integer' to myBB?)
I also see that the plug-in explicitly excludes IPV6 and just quietly "fails open" if it is asked to check an IPV6 IP address. I believe the service that the extension calls says IPV6 in partially implemented so that may be why. I suppose that flag can be omitted if someone wants to try. I don't have an IPV6 MyBB server to check.
I am also thinking of moving the check from "member_register_start" to "member_do_register_start". This is less user friendly since the new user has to fill in the form before being told to stop using a proxy. However, it also means that the user will have had to solve the captcha, and security question and this should greatly cut down on the number of calls to the service. A limitation of this simple plug-in is that it ignores error codes from the service such as being banned for exceeding query limits, so I think it's best to try to minimize queries caused by bots or angry spammers.
Other than those little issues, all seems well. It appears to work on a local test system. (With the caveat that my local test system always gets back a -3 response meaning: private address - which the plug-in doesn't check - but if I hack in a known VPN address to try, then it behaves as expected.)