I'm not a LUA coder myself, but even if it was possible to make a script that makes the UI think a certain key is being pressed, you'd still just be wasting time and effort since you're essentially looping through needless stuff.
Depending on the bag AddOn in question, you should be able to figure out what function makes it's bag GUI open up. Then you can use that function instead of Blizzard's OpenAllBags()