You can use the CSMB account as any other account but it is only valid for people living in the country.
Oof -- that violates the core purpose of the EU. The whole point is for Europeans to be on equal footing with respect to commerce. All European banks discriminate on the basis of nationality but in principle only against non-Europeans. Are you certain a Belgian cannot open a bank account in Portugal? This makes the prospect of forced-banking even more oppressive than I realised.