Generate non-identity primary key - linq-to-sql

Question

My workplace doesn't use identity columns or GUIDs for primary keys. Instead, we retrieve "next IDs" from a table as needed, and increment the value for each insert.
Unfortunatly for me, LINQ-TO-SQL appears to be optimized around using identity columns. So I need to query and update the "NextId" table whenever I perform an insert. For simplicity, I do this during the creation of the new object:
var db = new DataContext( "...connection string..." );
var car = Car
{
Id = GetNextId<Car>( db ),
TopSpeed = 88.0
};
db.InsertOnSubmit( car );
db.SubmitChanges();
The GetNextId method is something like this:
public int GetNextId<T>( DataContext db )
{
using ( var transaction = new TransactionScope ( TransactionScopeOption.RequiresNew ) )
{
var nextId = (from n in db.GetTable<NextId> ()
where n.TableName == typeof(T).Name
select n).Single ();
nextId.Value += 1;
db.SubmitChanges ();
transaction.Complete ();
return nextId.Value - 1;
}
}
Since all operations between creation of the data context and the call to SubmitChanges are part of one transaction, do I need to create a separate data context for retrieving next IDs? Each time I need an ID, I need to query and update a table inside a transaction to prevent multiple apps from grabbing the same value. The call to SubmitChanges() in the GetNextId() method would submit all previous operations, which I don't want to do.
Is a separate data context the only way, or is there something better I could try?