Composite Primary Key efficiency as a Foreign Key
The cost of using a simple synthetic integer PK is small, and the benefit in your case would probably be quite considerable.


As you point out, you'll have a much simpler FK relationship.
A small PK makes for small (and fast) indices.  Your total table space will probably be made less by adding such a column.
If business rules ever change, you won't have to reorder the table.


The only material downside that comes to mind is that you may lose performance on queries which benefited from clustering on the composite PK.  If you think that's likely to be significant, than continue clustering on the composite candidate key, but put the PK on the synthetic key.