With respect to VINs, I have this odd cognitive disconnect between "Isn't that personal information" and "Well, it's posted there on the front of your bike frame for anyone to see". I literally can't think of a single reason why it should be considered personal information, but that's the neuron bucket it is filed under in my brain for some reason.
As far as verifying ownership, I'm sure if someone were truly motivated they could hang back and pick a number that seems to have gone unclaimed if they really wanted to pretend.
On the Tesla forums they are tracking VINs (and paid deposit numbers) for largely the same reason (tracking volume numbers), and they spend a fair bit of time wondering about why various deliveries got various VIN numbers. It could appeal to my OCD Math Geek side, but I kind of feel like it is worrying about why different leaves fall face up or down.
None of that is meant to be any rhetorical argument one way or another, just random musings on what's going on in my brain.
Would this be a forum feature tied to their forum membership or an external site independently managed?
I would love to see the map of owners be popular, though, so anything that can be done to make it more inviting and not intimidating would be a plus in my book. I think zip code, name (first, last, or whatever they want to put) and forum handle and optional purchase date and optional VIN would make a good set of data. They should feel free to leave just their name or their forum handle or both, just something to refer to them by - unless it is a forum feature and thus their handle would be a given.
Contact info could be handled by encouraging them to sign up for the forum (unless it is a forum feature directly) and using the PM feature.
Just my 2c offered with no restocking fee...