Interesting, indeed. I have some remarks, though.

"The Belgians wear blue faced with orange" He must refer to militia units. Only one line bataillon still wore orange (the Nr 9) and this was not part of the Army of Occupation. Neither were any Belgian militia units, as far as I know. Therefore he saw Dutch, not Belgian militia.

Secondly, he describes the Nassau troops as having orange facings. This is certainly incorrect: The 2nd Nassau Light Infantry ware black facings and yellow piping. And the only plumes within the regiment were the scarlet of the grenadier caps. They did wear orange sashes though.

Was he writing from memory?