I have a HDR dressage monoflap, and regarding ducks question, I think it is noticeably closer feeling. Mine has pretty big knee rolls, and I feel like my leg is much tighter and slimmer to the horse than in my standard two flap dressage saddle. It really feels closer to bareback from my knees down.
And Edmonton, my billets are attached to the underside, the more forward one attaching halfway down near the knee roll and the back billet attaching under the seat like normal. Like this, but mine does not have that 'sleeve' over the top of the billets. monoflap dressage saddle - Bing Images
So you can visually inspect them, but I use a slightly thicker pad to make sure they don't pull against my horse's back weird, he seems content and doesn't work differently (well, he works better because I ride better!).