I don't know if you are still reading this, since your post is a few weeks old...
I haven't had much luck blocking off any opening much bigger than a doorway, for the reasons you're describing. Our old house had a big opening, and our new house has a 12 foot opening that I've blocked off. I'm using a Summer extendable gate, but can only anchor it on one side because there are only staircase spindles on the other side. (I've got it secured around the spindle with velcro loops on that side) Fortunately, it really only serves to keep the dog off the carpet/keep DD#2 contained for a minute while I run to the bathroom. I think I would also try to individually block off the staircases if possible, if I were in your situation.
Catherine
Mom to:
DD#1 3/07
DD#2 10/08
and "Bonus Mom" to:
DSD
DSS#1
DSS #2