We aren't going to host the public models because of the legal issues, and we don't plan to make a site for sharing them, but Outerra servers will function as trackers for the models. That means when someone imports a vehicle, it gets a unique identifier and a record about it can be uploaded to the server (must be if he means to use it in multiplayer). The record will contain the information needed to display some basic info about the vehicle like the name, a thumbnail image, a bounding box or bounding geometry, together with a user-provided link where the model can be downloaded.
If you encounter someone in a vehicle for what you don't have a model (and you would see just a labeled bounding geometry in the game), it can be downloaded automatically for you as long as the model file is reachable and can be downloaded. It would be best to support some services that enable anyone to put their models somewhere, for example the Google Drive. So in theory, when uploading the model info it could ask you if it should be uploaded to one of the supported hosting services, or you would provide a link to it. However, since the hosting usually requires an user interaction (clicking on the link), a web window would pop up if the file cannot be downloaded directly.
We are going to have a special extension for the compressed package files, and in our internal browser there will be an automatic url handler, meaning that if you click on a link pointing to a download of the package file, the engine would recognize it automatically and install the model(s) for you.
It also opens the possibility for someone to make a sharing site where people could put up their models, and the site could be either browsed from within the engine through its internal browser or through an external one, launching OT to install the models when an OT package file was downloaded.