What I meant by a flag is to actually add a new field to the products table to control this ...

That would require customizing the database table products and customizing the Admin to include this field for adding/editing and to then utilize this field in the Catalog ...

If you have a lot of products like this, then adding the new field is the method to use ...

If you only have a handful of Products that need this, then you could just customize the code to validate the shopping cart against a list of products_id and verify that the shipping address is allowed to have them ...

About how many Products are there like this?

Do they all belong to the same master_categories_id and do all Products using that master_categories_id have this limitation on shipping?