Thanks for the reply Sam. From what I can see it appears that the Prussian cavalry is decidedly inferior to all of the other lists (post Conquest period). In the Hundred Days period they can only take the Light Cavalry Abteilung (fair enough as there were no Cuirassiers around). There seems to be a possible option to replace a Dragoon or Hussar with an Uhlan but nothing official.
Personally I really like the idea of the Landwehr Cavalry but feel that they should be treated just like their foot slogging counterparts and rated as unpredictable. There doesn't seem to be any reason why the Landwehr cavalry would be any different in their motivation or skill. This would hardly change anything but would give them a chance of something useful and characterful. An interesting account is given below that certainly seems to indicate "unpredictability":
On 27th June 1815 the 1st Pomeranian Landwehr Cavalry Regiment moved to Senlis. They found the town empty of French troops. Hardly had the regiment dismounted on the market place to bivouac when an outpost returned in full flight. Hot on its heels was a detachment of French curiassiers from Donop's brigade (Kellermann's Corps). The landwehr had no time to remount and the commander of the cuirassiers demanded their surrender. Major Blankenburg decided otherwise. Leading those men who were mounted, he charged the French, taking advantage of the fact that they were bottled up in one of the streets and thus unable to manoeuvre.
The landwehr cut down the front rank of the cuirassiers and drove them out of the town. A detachment of 2nd Silesian Hussars supported the landwehr with carbine fire. Meanwhile, the elite French horse carabinier brigade arrived and the landwehr quickly fell back.
(Hofschroer - "1815: The Waterloo Campaign" p 230)
Does anyone think this could be a good idea?