But Tim, all of the mechanisms you mention (other than IP tracking) are designed to keep out Bad Guys, for whom the rightful owner wishes to deny access. In this situation, we have to instead stop what's known as an Insider Attack---the rightful owner *wants* to grant access to an outsider. Any authentication creditials can be passed by phone rather than email, and the insider (owner) has a financial interest in disclosing the information. Insider attacks are almost as hard as denial-of-service attacks (and you could argue that they are harder.) For example, if they do a callback, I can just list the broker's phone number as one that can be called.
And, personally, I'd trust your security policies at dvcnews.com before I'd trust the folks who maintain Disney's IT infrastructure, just based on the past history of "accomplishments" by the latter.
IP address matching is generally used as an access-granting technique rather than an access-denying one. Trying to block access to specific IP addresses is a losing proposition. It's neither hard nor expensive to route through indirection proxies. You just need a proxy, and there are plenty of them available. Even if it costs a few bucks a month to use it, it's going to be lost in the wash at
DVC rental prices at any volume at all.
If the broker wants to pay my dues, s/he is welcome to do so.
Edited to add: this came to mind in part because Wyndham recently disallowed all transfers between owners. This was intended in part to solve a point-washing problem (which it does solve) but also to make renting more difficult. The Wyndham mega-renters responded by doing something similar to what I'm suggesting will happen here---they just obtain login credentials for owners wishing to dispose of points. The Wyndham web site has much of the same financial information, etc. So, there is an existence proof that people may not mind disclosing that information when a little money can be made.