I personally can't see an issue with storing the make, model, regitration number and engine type in a register that's viewable by club members. All of that information could be found if the car was driven down a public highway and someone do a vehicle check using one of the many 'MOT history check' apps etc that are readily available on smartphones. The only thing they wouldn't get from that would be the owner's name.

I suppose there is a slight worry of theft by someone using the owner's name to find an address on an online phonebook - especially if the owner is a member of the forum, and posts their hometown on the members map. I can't imagine these cars get stolen very often (if ever???), due to the fact that they'd be pretty tricky to sell on. That said, I hear of plenty of Vincent motorcycles being stolen and they never seem to resurface, even in parts!