Legal provisions for optional referendums at national level

Brunei Darussalam
Answer
No
Source
Constitution of Brunei Darussalam
http://www.wipo.int/wipolex/en/details.jsp?id=7797
Direct Democracyin Asia: A Reference Guide to the Legislation and Practice (2005), page. 87 on Brunei.
http://www.docstoc.com/docs/74258274/Direct-Democracy-in-Asia
Constitutionalism in SoutheastAsia (KAS, 2008)
http://www.kas.de/wf/doc/kas_21147-1522-1-30.pdf?110815050450
Comment
The 1959 Constitution did not assist any democratic element to take root in the political governance in Brunei. Brunei remained an absolute monarchy, unconstrained by parliamentary institutions, or the judiciary.