I suspect that the likely reason you haven't heard anything about bags, other than the Vario bags, being able to utilize the BMW mounts is because there are none.
As to whether the side or top opening will work best for you, it's a personal decision. They each have there appeal. One of the reasons I have most often used the side opening design is that if you use top openers and you have a bag laid across the rear seat and strapped to the tops of the panniers you have to remove it in order to access the panniers. Typically this is not the case with the side opening design.
Another thing that I have noticed about certain top openers is that unless the lid is attached via a fixed hinge there is a danger that you will lose it, sometimes while you are on the road and before you realize what has happened. I hate backtracking for stuff that falls off of the bike! I have seen this happen several times. Some manufacturers provide or offer a leash to guard against this eventuality. However if you use a fixed leash you lose the nice feature of top openers of having the lid available around the campsite to provide a secure and stable tray that you can move away from the bike. In this case you should consider a quick release attachment to the box. Something like a light-weight steel cable and a small carabiner. Some models with a fixed hinge lid offer the ability to provide a secure and stable place to put a camp stove, park your coffee cup, etc. The lid can make a nice table. This can be a nice feature at a primitive campsite where you do not have a picnic table. I suppose the best of both worlds would be a hinged lid that is easily uncoupled but I do not know if anyone makes this.
The Vario bags are pretty much alone in their ability to adjust their size to your load which is a nice feature. as to things falling out when the lids are opened the use of multiple internal fleece bags (Aerostich) or clear zip lock bags will go a long way in helping you to stay organized and to keep your items in the panniers. The zip locks offer the extra margin of keeping things dry. I have used the BMW vario bags for 10 plus years and have never had them to leak but the belt and suspenders method makes me feel secure particularly with clothes, cameras, laptops, etc.
Like I said, it's a personal decision based on trade offs but, to answer your question, I would bet a dollar to a doughnut that there is no aftermarket bag offered that will mount on the BMW bag mounts.
LXDDLE screwed with this post 01-23-2013 at 10:43 AM