Hello Colin
First, for the tall rye grass. I suggest calling in at a sporting goods store that has fly tying supplies and ask for deer hair. It should be available in natural and dyed colors on a patch of hide. It can be painted before or after installation.
Second, I think your 3rd Kings German Legion Dragoon is really a 3rd Hussar, Kings German Legion. The Hussars jacket or dolman had horizontal lacing and 5 vertical rows of buttons, pointed cuffs with trefoils above, no epaulletts and lacework from waist to shoulder on the backside, a yellow and red barrelled sash.The pellisse (hanging jacket) would be discarded to the baggage before action.
Dragoons had layer cake looking sash. Epaulletts, plastron front and pointed cuffs on the jacket. Got this out of "Regiments at Waterloo" by Rene North and "Uniforms of Waterloo" by Haythornthwaite. I am not a rivet counter.
Cheers Gord