I would suggest finding or making either a steel or hardwood tapered rod and driving it or pushing it -as gently and carefully as you can, of course- into the seat tube. This will open up the area that needs to be opened without marring either the bolt ears or the slot and maintain the roundness of the seat tube opening. If you can find a piece of hard plastic, such as Delrin, this works good too.
Another method that works, but is usually my second choice, is a tapered chisel, such as you would find at sears or any good hardware store. They are usually hard chromed which makes for a slippery surface and won't mar the slot. Being very careful, insert the tool into the slot from the rear of the seat tube, perpendicular to the seat tube, and just by pushing the punch into the slot you may get some expansion. The critical thing to look for is the amount of taper to the tool. Too much and it may force that section of seat tube out of round. Too little taper and it will hit the opposite wall of the seat tube before you get any expansion in the slot.
I have used the screwdriver prying method in the past and never been happy with how it turns out. I have always had pry marks. Of course it may work just fine for you.
My .02 cents.