Just wondering, is it possible for entities created via model/DB first to implement an interface? If so, would it just take adding it to a partial class, like so:
public partial class SomeEntity : ISomeInterface
{
// blah
}
Or would it be more involved?
Yes, the approach you suggested should work normally. Alternatively you can use custom T4 templates to generate your classes with the interface implementations already present (via the template).