As this is starting at Waterloo I have checked on some details from the Waterloo Letters (including the Glover Unpublished ones).
Lord Greenock WL#7 mentions the intention to form Divisions but with the movements of the cavalry ordered on the 16th then this had not taken effect.
Greenock was ADC to Anglesey and was with him throughout the 18th "except when conveying his orders". These include taking orders to Vivian and Vandeleur to move to the right of La Haye Sainte in the afternoon.
Greenock also describes Anglesey communicating personally with the commander of the Belgian cavalry.
Thornhill (WL#8) describes taking verbal orders to the brigade commanders to act discretionally within certain constraints. This is correlated by Vandeleur (VL#51) who reports "an order arrived from Anglesey ….to engage the Enemy whenever they could do so with advantage without waiting for orders".
Thornhill also describes Anglesey had placed himself at the "torrent of attack". Thornhill was sent with orders for the Blues to advance as the charge of the Horseguards and Union Brigade took place.
Seymour (WL#9), another ADC to Anglesey, describes being sent to halt the Cumberland Hussars then returning and reporting the response to Anglesey. Seymour is then ordered to to deliver the message to the C/O of the Cumberland Hussars.
Seymour was later tasked to deliver the order to Vivian to move towards the centre.
Clark-Kennedy Royal Dragoons, Ponsonby's Brigade (WL#36). The Marquess of Anglesey came up at speed , Wheeled the Royals and Inniskillings into line and ordered them to charge, the Greys forming a second line in support.
As for Anglesey himself. WL#3 – On the morning of the 18th the Duke of Wellington said "The Prince of Orange requests the you will take charge of all his Cavalry". … "it is unfortunate that I should not have had the opportunity of making myself acquainted with any of his Officers or their Regiments".
On the 16th Anglesey first gallops to Quatre Bras, then hastens the march of the cavalry riding from Ninove. On the 17th he is "hands on " by positioning cavalry during the action at Genappe and ordering attacks.
WL#5 Anglesey – The charge against D'Erlon- "I observed very large masses of the Enemy…moving upon our left…..I immediately galloped to the Heavy Cavalry and ordered the Household Brigade to prepare to form line, passed on to Sir W Ponsonby's and having told him to wheel into line when the other Brigade did, I instantly returned to the Household Brigade and put the whole in motion".
Anglesey states later in the letter
"This forces from me the remark that I committed a great mistake in having myself led the attack. The carriere once begun, the leader is no better than any other man; whereas, if I had placed myself at the head of the 2nd line, there is no saying what great advantages might not have accrued from it".
It should be noted that Vandeleur's support for Ponsonby was very much on Vandeleur's initiative in line with his orders and freedom of action.
So I conclude that co-ordination is very much down to the senior commander at the point of action. Kellerman at Quatre Bras seems to have used a similar approach. ADC may be sent off with orders for other brigades (or divisions) but the senior commander will want to place himself with the commanders of the divisions that are yet to be committed and order these brigades how he wishes.
Another good justification for the French cavalry column (perpendicular approach) where the commander of the column can position himself to order the leading regiments to deploy into line and charge then ordering the following units to reinforce the attack or deploy as supports to allow the first regiments to rally.
The battle of Liebertwolkwitz (14th October 1813) sees the French using deep columns with Murat bringing up divisions from the rear to continue the fight whilst the allied cavalry arrived and attacked in a somewhat ad-hoc manner eventually pushing the French back. There is no indication of any exceptional co-ordination by the Allies in this.
How to reflect this in rules? I think the simplest way is to identify a cavalry commander for the army separate from the CinC and use a parallel orders system, activation or whatever when the cavalry commander is co-located with the cavalry Division commanders.