I'm glad that the possible parallel I raised seems plausible. I think it would be worthwhile to look into other battles where Napoleon was present or other heads of state.
Napoleon's personal staff was huge as was Berthier's. I am not sure of Soult's in 1815, but it would not be small. Few of these personnel would be needed on any battlefield, and would probably be busy with work as usual. During a battle they would need to be protected against any roving threat. Any head of state would have some kind of retinue present.
In general, all armies would have staff, critical baggage (such as the treasury) and an artillery park. So how was this protected?
I think that the bataillon de Neufchatel was allocated to this duty in 1812, but would need to dig into OOBs for other campaigns.
Coming back to Waterloo, I think the battalion allocated to this guard duty was committed to action later in the day. There is a record of Napoleon's carriage being captured, but not the treasury. So at some point in the day, someone must have given the order for the staff and some part of the HQ baggage to move to the rear. I cannot recall any mention of this.
At Vittoria, Graham's column cut the main road north, trapping the baggage. There are extensive accounts of Allied troops looting it. That it was cut off seems to have been somewhat rare.
I'm curious to see where your inquiry leads,