Willem Frederik was an army man. He followed the Prince in 1795 and served for a time in the British Army. He left the service in 1802 and returned to his possesions in Holland (Breda). In 1813 he returned to active duty to serve the new independant Dutch State.
These old biographical standard works don't mention any public activity between 1802 and 1813. The Iconographisch Bureau and Schutte state that he was appointed Royal Equerry to King Louis Napoleon in 5 August 1809, but that's not mentioned in the above publications, Only in the Yearbook of the Dutch Nobility and the Dutch Book of Nobility and I think this was a question of mistaken identity. Carel Adam van Bylandt was appointed ADC to the King with the rank of Lieutenant-Colonel on 25 August 1809.
The Bylandt of this portrait wears the dress of a state official, not a military uniform, and Carel Adam was a Commander as of 1 January 1810, so it can't be him. Two other Bylandts were in the service of the Kingdom of Holland. Otto Anne (Queen's Chamberlain) was a Commander in the Order from 1808. Frederik Sigismund (President of the 'Lawgiving Body' from 1808) was a Grand Cross from 5 june 1810.
Jean Charles, the brother of Willem Frederik had also left Dutch Service in 1795 and did not serve in the military after that, but was an emmissary from 1807. He was appointed Knight in the Order of the Union on 10 March 1807. He was not promoted in the order.
Another point: the Order of the Union had an 8-pointed star, that of the Reunion a 12-pointed one. The subject of the miniature clearly wears an eight-pointed star.