shegy,
Yes, I have seen the EDD add-on. That's why I said "most other membership plugins"! But it just isn't necessary.
What any tax authorities will require from you is that your accounts seem genuine (all auditing involves sampling anyway, not checking every single item) and are internally consistent. They aren't going to be checking whether a particular IP address exists or not. In any event, I'm sure they recognize that IP tracking is imperfect.
AS I said, most IP "inaccuracies" are perfectly legitimate. (No tax authority is going to hold you liable for someone else's fraud anyway, unless you colluded in it.) IP "inaccuracies" can be caused, for example, simply by the location of an internet service provider. If that provider has a base in a different country, then the IP will be "wrong". Or if the computer was purchased in a different country, that can sometimes cause a similar result. I'm sure you've seen all this, and more, with Stat Tracking plugins.
So the question is: what can a plugin extension actually offer? It can't claim to capture "accurate" IP addresses, because it would be subject to the same constraints as any other stat tracking software. It could ask the user to input his or her address again -- say, one for billing address, and one for delivery -- but s2Member forms already enable you to do that.
So I just can't see what a plugin extension could offer, unless, like Quaderno, it was going to calculate the VAT for every country for you. But that's not really membership plugin territory.