There are indeed certain special diplomatic or tax regimes that can exempt you from inheritance tax.

Only officials of the Institutions of the European Union, the European Investment Bank, NATO, the Western European Union, etc ... who are living in Belgium for the purpose of carrying out their duties are considered to be domiciled in their country of origin for inheritance tax purposes.  If these officials are not domiciled in Belgium, their heirs will pay Belgian inheritance tax but only on real property in Belgium.

Other special tax regimes (such as the expatriate tax regime) do not give an exemption of inheritance tax in Belgium, unless the expatriate had not taken up domicile in Belgium.