Did you have trouble with some certain bag/mount before? I don`t think anybody makes them specifically for riser or drop bars. I have Cannondale mounts on my mountain bike (with riser bars), my wife`s "old lady" comfort bike (also with riser bars), and on our tandem (with drops). The same combination of big bag or little bag will fit on any of those bikes. I did have to get a little creative with cable routing for my wife`s bike to make it work. The Canondale system is the only comercial bag/mount setup I`ve ever used, but from looking at other manufacturers stuff on the web and in the stores, I`d venture to guess they all have pretty much the same issues and requirements. If in doubt, try a cheap one (or borrow one first) so you won`t be out much. Getting around the cables is the biggest potential problem, but you ought to be able to figure out a way by moving them around or tilting the bag.
Horizontally fixed? Yes in that they`re always the same distance apart. If I understand your concerns, there`s plenty of room between the two clamps for your stem, but they still fit quite well between the curves of your bars, so they don`t end up trying to clamp onto the curved parts.
Or is it the rotating part that throws you? In that respect, they don`t lock into place. You can rotate them up or down as needed (before you clamp them tight) and supposedly they won`t slip down (after you snug down the clamp bolts) with the weight of the bag. In practice, I`ve found that the Cdale mounts do indeed slip down with time, but that can be overcome easily. On one bike, I mounted a little aluminum "finger" to hold up the bottom of the mounting plate, on the others I found that putting a little piece of inner tube between the bar and the clamp gives it plenty of grip.
If you still have doubts, I suggest going to a bike shop that carries the bags you`re interrested in and ask them if you can try it out in the store and on your own bike to make sure everything is compatible. You won`t have the kind of selection that`s available over the internet, but you will be absolutely sure that the product you buy will be useable- that`s the advantage of buying stuff from your LBS. Honestly, I doubt you have anything to worry about- the boys at your bike shop will be able to fix you up with something and explain it all to you with real products in hand and with your own bike.