Hi there, your idea and intensions are great.
What does amaze me is how quickly some people shout out the negative aspects and not offer a solution.
Instead of a standing data base of owned vehicles, you could limit it to a data base of stolen vehicles, with a police case ref. no. and contact tel. no. police/owner, thus should anyone suspect a particular vehicle they could search the data base. (so no you are not inviting the thieves to your home as the car is already stolen)
We all see individual messages of my LR is gone, but a week later try find that message again, all these messages need to be collected in one folder.
As far as data protection you would be consenting to putting these details in a public domain.
In addition you could link it to an automated email system to go out to forums whom would post it on their site, and it can be linked to the text check system as suggested.
You are right in saying we should stand united against crime.