Here are some ideas...
Bungee cords with hooks would be quicker to hook and unhook, but less secure and put more stress on the panel itself. Perhaps using wire on one side and bungee cords on the other? Then you could just unhook from the bungee side.
Wide velcro straps can be very, very secure.
You could also make a high shelf out of pvc pipe or pvc shelving, or just use a lighweight narrow table like for patio or camping use. This would make a shelf that's hopefully high enough to allow easy access to the dry bag.
What about mounting a rear rack on the yak wheel? Some racks use skewer mounts and then a small stabilizing rail or rails (or straps) could be used in the front. And if the front stabilizer could sit behind the day bag, then you's have very good access to the bag.
Another thought. If you are just interested in easy access to the bag zipper. Would it be possible to lay the bag on its side so that the zipper is to the left or right?
Found this photo on a google search.
